diff --git a/src/d_main.cpp b/src/d_main.cpp
index cac75766f..cefd446b9 100644
--- a/src/d_main.cpp
+++ b/src/d_main.cpp
@@ -530,9 +530,6 @@ CVAR (Flag, sv_respawnsuper,		dmflags2, DF2_RESPAWN_SUPER);
 //
 //==========================================================================
 
-int i_compatflags, i_compatflags2;	// internal compatflags composed from the compatflags CVAR and MAPINFO settings
-int ii_compatflags, ii_compatflags2, ib_compatflags;
-
 EXTERN_CVAR(Int, compatmode)
 
 static int GetCompatibility(FLevelLocals *Level, int mask)
@@ -549,17 +546,24 @@ static int GetCompatibility2(FLevelLocals *Level, int mask)
 
 CUSTOM_CVAR (Int, compatflags, 0, CVAR_ARCHIVE|CVAR_SERVERINFO)
 {
-	int old = i_compatflags;
-	i_compatflags = GetCompatibility(&level, self) | ii_compatflags;
-	if ((old ^ i_compatflags) & COMPATF_POLYOBJ)
+	for (auto Level : AllLevels())
 	{
-		level.ClearAllSubsectorLinks();
+		int old = Level->i_compatflags;
+		Level->i_compatflags = GetCompatibility(Level, self) | Level->ii_compatflags;
+		if ((old ^ Level->i_compatflags) & COMPATF_POLYOBJ)
+		{
+			Level->ClearAllSubsectorLinks();
+		}
 	}
 }
 
 CUSTOM_CVAR (Int, compatflags2, 0, CVAR_ARCHIVE|CVAR_SERVERINFO)
 {
-	i_compatflags2 = GetCompatibility2(&level, self) | ii_compatflags2;
+	for (auto Level : AllLevels())
+	{
+		Level->i_compatflags2 = GetCompatibility2(Level, self) | Level->ii_compatflags2;
+		Level->SetCompatLineOnSide(true);
+	}
 }
 
 CUSTOM_CVAR(Int, compatmode, 0, CVAR_ARCHIVE|CVAR_NOINITCALL)
diff --git a/src/doomdata.h b/src/doomdata.h
index ee0b21f32..05bcfbfc1 100644
--- a/src/doomdata.h
+++ b/src/doomdata.h
@@ -151,7 +151,9 @@ enum ELineFlags : unsigned
 	ML_MAPPED					= 0x00000100,	// set if already drawn in automap
 	ML_REPEAT_SPECIAL			= 0x00000200,	// special is repeatable
 
+	// 0x400, 0x800 and 0x1000 are ML_SPAC_MASK, they can be used for internal things but not for real map flags.
 	ML_ADDTRANS					= 0x00000400,	// additive translucency (can only be set internally)
+	ML_COMPATSIDE				= 0x00000800,	// for compatible PointOnLineSide checks. Using the global compatibility check would be a bit expensive for this check.
 
 	// Extended flags
 	ML_MONSTERSCANACTIVATE		= 0x00002000,	// [RH] Monsters (as well as players) can activate the line
diff --git a/src/doomstat.h b/src/doomstat.h
index 6f0007f4c..23356b25f 100644
--- a/src/doomstat.h
+++ b/src/doomstat.h
@@ -248,7 +248,6 @@ EXTERN_CVAR (Int, dmflags2);	// [BC]
 
 EXTERN_CVAR (Int, compatflags);
 EXTERN_CVAR (Int, compatflags2);
-extern int i_compatflags, i_compatflags2, ii_compatflags, ii_compatflags2, ib_compatflags;
 
 // Filters from AddAutoloadFiles(). Used to filter files from archives.
 extern FString LumpFilterIWAD;
diff --git a/src/dsectoreffect.cpp b/src/dsectoreffect.cpp
index adc9c5f0a..8fbf3c8a7 100644
--- a/src/dsectoreffect.cpp
+++ b/src/dsectoreffect.cpp
@@ -214,7 +214,7 @@ EMoveResult sector_t::MoveFloor(double speed, double dest, int crush, int direct
 		//destheight = (dest < ceilingheight) ? dest : ceilingheight;
 		if (!ceilingplane.isSlope() && !floorplane.isSlope() &&
 			!PortalIsLinked(sector_t::ceiling) &&
-			(!(i_compatflags2 & COMPATF2_FLOORMOVE) && -dest > ceilingplane.fD()))
+			(!(Level->i_compatflags2 & COMPATF2_FLOORMOVE) && -dest > ceilingplane.fD()))
 		{
 			dest = -ceilingplane.fD();
 		}
@@ -304,7 +304,7 @@ EMoveResult sector_t::MoveCeiling(double speed, double dest, int crush, int dire
 		//destheight = (dest > floorheight) ? dest : floorheight;
 		if (!ceilingplane.isSlope() && !floorplane.isSlope() &&
 			!PortalIsLinked(sector_t::floor) &&
-			(!(i_compatflags2 & COMPATF2_FLOORMOVE) && dest < -floorplane.fD()))
+			(!(Level->i_compatflags2 & COMPATF2_FLOORMOVE) && dest < -floorplane.fD()))
 		{
 			dest = -floorplane.fD();
 		}
diff --git a/src/g_level.cpp b/src/g_level.cpp
index 8bc9ab97d..5491367ad 100644
--- a/src/g_level.cpp
+++ b/src/g_level.cpp
@@ -578,7 +578,7 @@ void G_ChangeLevel(const char *levelname, int position, int flags, int nextSkill
 		Printf (TEXTCOLOR_RED "Unloading scripts cannot exit the level again.\n");
 		return;
 	}
-	if (gameaction == ga_completed && !(i_compatflags2 & COMPATF2_MULTIEXIT))	// do not exit multiple times.
+	if (gameaction == ga_completed && !(level.i_compatflags2 & COMPATF2_MULTIEXIT))	// do not exit multiple times.
 	{
 		return;
 	}
@@ -1070,7 +1070,7 @@ void G_DoLoadLevel (const FString &nextmapname, int position, bool autosave, boo
 	level.starttime = gametic;
 
 	level.UnSnapshotLevel (!savegamerestore);	// [RH] Restore the state of the level.
-	int pnumerr = G_FinishTravel ();
+	int pnumerr = level.FinishTravel ();
 
 	if (!level.FromSnapshot)
 	{
@@ -2075,6 +2075,21 @@ int FLevelLocals::GetInfighting()
 	return G_SkillProperty(SKILLP_Infight);
 }
 
+//============================================================================
+//
+// transfers the compatiblity flag for old PointOnLineSide to each line.
+// This gets checked in a frequently called worker function and the closer
+// this info is to the data this function works on, the better.
+//
+//============================================================================
+
+void FLevelLocals::SetCompatLineOnSide(bool state)
+{
+	int on = (state && (i_compatflags2 & COMPATF2_POINTONLINE));
+	if (on) for (auto l : lines) l.flags |= ML_COMPATSIDE;
+	else for (auto l : lines) l.flags &= ML_COMPATSIDE;
+}
+
 //==========================================================================
 // IsPointInMap
 //
diff --git a/src/g_levellocals.h b/src/g_levellocals.h
index 758370db5..aa19b989e 100644
--- a/src/g_levellocals.h
+++ b/src/g_levellocals.h
@@ -120,6 +120,7 @@ struct FLevelLocals
 	void SetConversation(int convid, PClassActor *Class, int dlgindex);
 	int FindNode (const FStrifeDialogueNode *node);
     int GetInfighting();
+	void SetCompatLineOnSide(bool state);
 	void Init();
 
 private:
@@ -442,6 +443,12 @@ public:
 	FDialogueIDMap DialogueRoots;
 	FDialogueMap ClassRoots;
 
+	int ii_compatflags = 0;
+	int ii_compatflags2 = 0;
+	int ib_compatflags = 0;
+	int i_compatflags = 0;
+	int i_compatflags2 = 0;
+
 	uint8_t		md5[16];			// for savegame validation. If the MD5 does not match the savegame won't be loaded.
 	int			time;			// time in the hub
 	int			maptime;		// time in the map
diff --git a/src/hwrenderer/scene/hw_drawlist.cpp b/src/hwrenderer/scene/hw_drawlist.cpp
index f50c49b8b..b6f4859f1 100644
--- a/src/hwrenderer/scene/hw_drawlist.cpp
+++ b/src/hwrenderer/scene/hw_drawlist.cpp
@@ -588,7 +588,7 @@ inline int HWDrawList::CompareSprites(SortNode * a,SortNode * b)
 	int res = s1->depth - s2->depth;
 
 	if (res != 0) return -res;
-	else return (i_compatflags & COMPATF_SPRITESORT)? s2->index-s1->index : s1->index-s2->index;
+	else return reverseSort? s2->index-s1->index : s1->index-s2->index;
 }
 
 //==========================================================================
@@ -702,6 +702,7 @@ SortNode * HWDrawList::DoSort(HWDrawInfo *di, SortNode * head)
 //==========================================================================
 void HWDrawList::Sort(HWDrawInfo *di)
 {
+	reverseSort = !!(di->Level->i_compatflags & COMPATF_SPRITESORT);
     SortZ = di->Viewpoint.Pos.Z;
 	MakeSortList();
 	sorted = DoSort(di, SortNodes[SortNodeStart]);
diff --git a/src/hwrenderer/scene/hw_drawlist.h b/src/hwrenderer/scene/hw_drawlist.h
index 66ae468ed..e4279516c 100644
--- a/src/hwrenderer/scene/hw_drawlist.h
+++ b/src/hwrenderer/scene/hw_drawlist.h
@@ -67,6 +67,7 @@ struct HWDrawList
 	int SortNodeStart;
     float SortZ;
 	SortNode * sorted;
+	bool reverseSort;
 	
 public:
 	HWDrawList()
diff --git a/src/hwrenderer/scene/hw_walls.cpp b/src/hwrenderer/scene/hw_walls.cpp
index 9b42ac5cc..91ec1a5f8 100644
--- a/src/hwrenderer/scene/hw_walls.cpp
+++ b/src/hwrenderer/scene/hw_walls.cpp
@@ -2068,7 +2068,7 @@ void GLWall::Process(HWDrawInfo *di, seg_t *seg, sector_t * frontsector, sector_
 		FTexture *tex = TexMan.GetTexture(seg->sidedef->GetTexture(side_t::mid), true);
 		if (tex != NULL)
 		{
-			if (i_compatflags & COMPATF_MASKEDMIDTEX)
+			if (di->Level->i_compatflags & COMPATF_MASKEDMIDTEX)
 			{
 				tex = tex->GetRawTexture();
 			}
diff --git a/src/maploader/compatibility.cpp b/src/maploader/compatibility.cpp
index 1c9ed5c60..f3a424cac 100644
--- a/src/maploader/compatibility.cpp
+++ b/src/maploader/compatibility.cpp
@@ -279,17 +279,17 @@ FName MapLoader::CheckCompatibility(MapData *map)
 
 	if (BCompatMap.CountUsed() == 0) ParseCompatibility();
 
-	ii_compatflags = 0;
-	ii_compatflags2 = 0;
-	ib_compatflags = 0;
+	Level->ii_compatflags = 0;
+	Level->ii_compatflags2 = 0;
+	Level->ib_compatflags = 0;
 
 	// When playing Doom IWAD levels force COMPAT_SHORTTEX and COMPATF_LIGHT.
 	// I'm not sure if the IWAD maps actually need COMPATF_LIGHT but it certainly does not hurt.
 	// TNT's MAP31 also needs COMPATF_STAIRINDEX but that only gets activated for TNT.WAD.
 	if (Wads.GetLumpFile(map->lumpnum) == Wads.GetIwadNum() && (gameinfo.flags & GI_COMPATSHORTTEX) && Level->maptype == MAPTYPE_DOOM)
 	{
-		ii_compatflags = COMPATF_SHORTTEX|COMPATF_LIGHT;
-		if (gameinfo.flags & GI_COMPATSTAIRS) ii_compatflags |= COMPATF_STAIRINDEX;
+		Level->ii_compatflags = COMPATF_SHORTTEX|COMPATF_LIGHT;
+		if (gameinfo.flags & GI_COMPATSTAIRS) Level->ii_compatflags |= COMPATF_STAIRINDEX;
 	}
 
 	map->GetChecksum(md5.Bytes);
@@ -319,9 +319,9 @@ FName MapLoader::CheckCompatibility(MapData *map)
 
 	if (flags != NULL)
 	{
-		ii_compatflags |= flags->CompatFlags[SLOT_COMPAT];
-		ii_compatflags2 |= flags->CompatFlags[SLOT_COMPAT2];
-		ib_compatflags |= flags->CompatFlags[SLOT_BCOMPAT];
+		Level->ii_compatflags |= flags->CompatFlags[SLOT_COMPAT];
+		Level->ii_compatflags2 |= flags->CompatFlags[SLOT_COMPAT2];
+		Level->ib_compatflags |= flags->CompatFlags[SLOT_BCOMPAT];
 	}
 
 	// Reset i_compatflags
@@ -330,7 +330,7 @@ FName MapLoader::CheckCompatibility(MapData *map)
 	// Set floatbob compatibility for all maps with an original Hexen MAPINFO.
 	if (Level->flags2 & LEVEL2_HEXENHACK)
 	{
-		ib_compatflags |= BCOMPATF_FLOATBOB;
+		Level->ib_compatflags |= BCOMPATF_FLOATBOB;
 	}
 	return FName(hash, true);	// if this returns NAME_None it means there is no scripted compatibility handler.
 }
@@ -574,6 +574,9 @@ CCMD (mapchecksum)
 
 CCMD (hiddencompatflags)
 {
-	Printf("%08x %08x %08x\n", ii_compatflags, ii_compatflags2, ib_compatflags);
+	for(auto Level : AllLevels())
+	{
+		Printf("%s: %08x %08x %08x\n", Level->MapName.GetChars(), Level->ii_compatflags, Level->ii_compatflags2, Level->ib_compatflags);
+	}
 }
 
diff --git a/src/maploader/maploader.cpp b/src/maploader/maploader.cpp
index aa4c2bd7c..a22491009 100644
--- a/src/maploader/maploader.cpp
+++ b/src/maploader/maploader.cpp
@@ -1516,7 +1516,7 @@ void MapLoader::SetLineID (int i, line_t *ld)
 			break;
 			
 		case Plane_Align:
-			if (!(ib_compatflags & BCOMPATF_NOSLOPEID)) setid = ld->args[2];
+			if (!(Level->ib_compatflags & BCOMPATF_NOSLOPEID)) setid = ld->args[2];
 			break;
 			
 		case Static_Init:
@@ -3016,7 +3016,7 @@ void MapLoader::LoadLevel(MapData *map, const char *lumpname, int position)
 		Level->maptype = MAPTYPE_UDMF;
 	}
 	FName checksum = CheckCompatibility(map);
-	if (ib_compatflags & BCOMPATF_REBUILDNODES)
+	if (Level->ib_compatflags & BCOMPATF_REBUILDNODES)
 	{
 		ForceNodeBuild = true;
 	}
diff --git a/src/maploader/slopes.cpp b/src/maploader/slopes.cpp
index 92c761103..2fb44e628 100644
--- a/src/maploader/slopes.cpp
+++ b/src/maploader/slopes.cpp
@@ -179,7 +179,7 @@ void MapLoader::SetSlope (secplane_t *plane, bool setCeil, int xyangi, int zangi
 
 	DVector3 norm;
 
-	if (ib_compatflags & BCOMPATF_SETSLOPEOVERFLOW)
+	if (Level->ib_compatflags & BCOMPATF_SETSLOPEOVERFLOW)
 	{
 		// We have to consider an integer multiplication overflow here.
 		norm[0] = FixedToFloat(FloatToFixed(zang.Cos()) * FloatToFixed(xyang.Cos())) / 65536.;
diff --git a/src/p_doors.cpp b/src/p_doors.cpp
index 00b4f214e..c3c8b92e8 100644
--- a/src/p_doors.cpp
+++ b/src/p_doors.cpp
@@ -352,7 +352,7 @@ void DDoor::Construct(sector_t *sec, EVlDoor type, double speed, int delay, int
 	m_TopCountdown = topcountdown;
 	m_LightTag = lightTag;
 
-	if (i_compatflags & COMPATF_NODOORLIGHT)
+	if (Level->i_compatflags & COMPATF_NODOORLIGHT)
 	{
 		m_LightTag = 0;
 	}
diff --git a/src/p_enemy.cpp b/src/p_enemy.cpp
index 17f1e8561..162e9a76c 100644
--- a/src/p_enemy.cpp
+++ b/src/p_enemy.cpp
@@ -464,7 +464,7 @@ int P_Move (AActor *actor)
 
 	// [RH] I'm not so sure this is such a good idea
 	// [GZ] That's why it's compat-optioned.
-	if (compatflags & COMPATF_MBFMONSTERMOVE && !(actor->flags8 & MF8_NOFRICTION))
+	if (actor->Level->i_compatflags & COMPATF_MBFMONSTERMOVE && !(actor->flags8 & MF8_NOFRICTION))
 	{
 		// killough 10/98: make monsters get affected by ice and sludge too:
 		movefactor = P_GetMoveFactor (actor, &friction);
@@ -864,7 +864,7 @@ void P_NewChaseDir(AActor * actor)
 	if (actor->floorz - actor->dropoffz > actor->MaxDropOffHeight && 
 		actor->Z() <= actor->floorz && !(actor->flags & MF_DROPOFF) && 
 		!(actor->flags2 & MF2_ONMOBJ) &&
-		!(actor->flags & MF_FLOAT) && !(i_compatflags & COMPATF_DROPOFF))
+		!(actor->flags & MF_FLOAT) && !(actor->Level->i_compatflags & COMPATF_DROPOFF))
 	{
 		FBoundingBox box(actor->X(), actor->Y(), actor->radius);
 		FBlockLinesIterator it(actor->Level, box);
@@ -1715,7 +1715,7 @@ int P_LookForPlayers (AActor *actor, INTBOOL allaround, FLookExParams *params)
 		// the player then, eh?
 		if(!(actor->flags6 & MF6_SEEINVISIBLE)) 
 		{
-			if ((player->mo->flags & MF_SHADOW && !(i_compatflags & COMPATF_INVISIBILITY)) ||
+			if ((player->mo->flags & MF_SHADOW && !(actor->Level->i_compatflags & COMPATF_INVISIBILITY)) ||
 				player->mo->flags3 & MF3_GHOST)
 			{
 				if (player->mo->Distance2D (actor) > 128 && player->mo->Vel.XY().LengthSquared() < 5*5)
@@ -1776,7 +1776,7 @@ DEFINE_ACTION_FUNCTION(AActor, A_Look)
 	}
 	else
 	{
-		targ = (i_compatflags & COMPATF_SOUNDTARGET || self->flags & MF_NOSECTOR)? 
+		targ = (self->Level->i_compatflags & COMPATF_SOUNDTARGET || self->flags & MF_NOSECTOR)? 
 			self->Sector->SoundTarget : self->LastHeard;
 
 		// [RH] If the soundtarget is dead, don't chase it
@@ -1909,7 +1909,7 @@ DEFINE_ACTION_FUNCTION(AActor, A_LookEx)
 	{
 		if (!(flags & LOF_NOSOUNDCHECK))
 		{
-			targ = (i_compatflags & COMPATF_SOUNDTARGET || self->flags & MF_NOSECTOR)?
+			targ = (self->Level->i_compatflags & COMPATF_SOUNDTARGET || self->flags & MF_NOSECTOR)?
 				self->Sector->SoundTarget : self->LastHeard;
 			if (targ != NULL)
 			{
@@ -2721,7 +2721,7 @@ bool P_CheckForResurrection(AActor *self, bool usevilestates)
 				{
 					corpsehit->Translation = info->Translation; // Clean up bloodcolor translation from crushed corpses
 				}
-				if (ib_compatflags & BCOMPATF_VILEGHOSTS)
+				if (self->Level->ib_compatflags & BCOMPATF_VILEGHOSTS)
 				{
 					corpsehit->Height *= 4;
 					// [GZ] This was a commented-out feature, so let's make use of it,
@@ -3116,7 +3116,7 @@ void A_BossDeath(AActor *self)
 						LEVEL_SORCERER2SPECIAL)) == 0)
 		return;
 
-	if ((i_compatflags & COMPATF_ANYBOSSDEATH) || ( // [GZ] Added for UAC_DEAD
+	if ((Level->i_compatflags & COMPATF_ANYBOSSDEATH) || ( // [GZ] Added for UAC_DEAD
 		((Level->flags & LEVEL_MAP07SPECIAL) && (type == NAME_Fatso || type == NAME_Arachnotron)) ||
 		((Level->flags & LEVEL_BRUISERSPECIAL) && (type == NAME_BaronOfHell)) ||
 		((Level->flags & LEVEL_CYBORGSPECIAL) && (type == NAME_Cyberdemon)) ||
diff --git a/src/p_floor.cpp b/src/p_floor.cpp
index 4f6e087b1..0bd2ae5b7 100644
--- a/src/p_floor.cpp
+++ b/src/p_floor.cpp
@@ -461,7 +461,7 @@ bool FLevelLocals::CreateFloor(sector_t *sec, DFloor::EFloor floortype, line_t *
 		// [Graf Zahl]
 		// Don't make sounds for instant movement hacks but make an exception for
 		// switches that activate their own back side. 
-		if (!(i_compatflags & COMPATF_SILENT_INSTANT_FLOORS))
+		if (!(sec->Level->i_compatflags & COMPATF_SILENT_INSTANT_FLOORS))
 		{
 			if (!line || !(line->activation & (SPAC_Use | SPAC_Push)) || line->backsector != sec)
 				silent = true;
diff --git a/src/p_lnspec.cpp b/src/p_lnspec.cpp
index ed594244b..04d7d9337 100644
--- a/src/p_lnspec.cpp
+++ b/src/p_lnspec.cpp
@@ -3065,7 +3065,7 @@ FUNC(LS_SetPlayerProperty)
 	{
 		int i;
 
-		if ((ib_compatflags & BCOMPATF_LINKFROZENPROPS) && (mask & (CF_FROZEN | CF_TOTALLYFROZEN)))
+		if ((Level->ib_compatflags & BCOMPATF_LINKFROZENPROPS) && (mask & (CF_FROZEN | CF_TOTALLYFROZEN)))
 		{ // Clearing one of these properties clears both of them (if the compat flag is set.)
 			mask = CF_FROZEN | CF_TOTALLYFROZEN;
 		}
diff --git a/src/p_map.cpp b/src/p_map.cpp
index 1d14c00bd..ebfda7f83 100644
--- a/src/p_map.cpp
+++ b/src/p_map.cpp
@@ -459,7 +459,7 @@ bool	P_TeleportMove(AActor* thing, const DVector3 &pos, bool telefrag, bool modi
 		// [RH] Z-Check
 		// But not if not MF2_PASSMOBJ or MF3_DONTOVERLAP are set!
 		// Otherwise those things would get stuck inside each other.
-		if ((thing->flags2 & MF2_PASSMOBJ || th->flags4 & MF4_ACTLIKEBRIDGE) && !(i_compatflags & COMPATF_NO_PASSMOBJ))
+		if ((thing->flags2 & MF2_PASSMOBJ || th->flags4 & MF4_ACTLIKEBRIDGE) && !(thing->Level->i_compatflags & COMPATF_NO_PASSMOBJ))
 		{
 			if (!(th->flags3 & thing->flags3 & MF3_DONTOVERLAP))
 			{
@@ -516,7 +516,7 @@ bool	P_TeleportMove(AActor* thing, const DVector3 &pos, bool telefrag, bool modi
 		// If this teleport was caused by a move, P_TryMove() will handle the
 		// sector transition messages better than we can here.
 		// This needs to be compatibility optioned because some older maps exploited this missing feature.
-		if (!(thing->flags6 & MF6_INTRYMOVE) && !(i_compatflags2 & COMPATF2_TELEPORT))
+		if (!(thing->flags6 & MF6_INTRYMOVE) && !(thing->Level->i_compatflags2 & COMPATF2_TELEPORT))
 		{
 			thing->CheckSectorTransition(oldsec);
 		}
@@ -850,7 +850,7 @@ bool PIT_CheckLine(FMultiBlockLinesIterator &mit, FMultiBlockLinesIterator::Chec
 	// into being friendly with the MBF flag automatically gain MF3_NOBLOCKMONST, so this
 	// just optionally generalizes the behavior to other friendly monsters.
 	bool NotBlocked = ((tm.thing->flags3 & MF3_NOBLOCKMONST)
-		|| ((i_compatflags & COMPATF_NOBLOCKFRIENDS) && (tm.thing->flags & MF_FRIENDLY)));
+		|| ((tm.thing->Level->i_compatflags & COMPATF_NOBLOCKFRIENDS) && (tm.thing->flags & MF_FRIENDLY)));
 
 	uint32_t ProjectileBlocking = ML_BLOCKEVERYTHING | ML_BLOCKPROJECTILE;
 	if ( tm.thing->flags8 & MF8_BLOCKASPLAYER ) ProjectileBlocking |= ML_BLOCK_PLAYERS | ML_BLOCKING;
@@ -1263,7 +1263,7 @@ bool PIT_CheckThing(FMultiBlockThingsIterator &it, FMultiBlockThingsIterator::Ch
 	// walking on other actors and unblocking is too messy through restricted portal types so disable it.
 	if (!(cres.portalflags & FFCF_RESTRICTEDPORTAL))
 	{
-		if (!(i_compatflags & COMPATF_NO_PASSMOBJ) && !(tm.thing->flags & (MF_FLOAT | MF_MISSILE | MF_SKULLFLY | MF_NOGRAVITY)) &&
+		if (!(thing->Level->i_compatflags & COMPATF_NO_PASSMOBJ) && !(tm.thing->flags & (MF_FLOAT | MF_MISSILE | MF_SKULLFLY | MF_NOGRAVITY)) &&
 			(thing->flags & MF_SOLID) && (thing->flags4 & MF4_ACTLIKEBRIDGE))
 		{
 			// [RH] Let monsters walk on actors as well as floors
@@ -1311,7 +1311,7 @@ bool PIT_CheckThing(FMultiBlockThingsIterator &it, FMultiBlockThingsIterator::Ch
 
 	// [RH] If the other thing is a bridge, then treat the moving thing as if it had MF2_PASSMOBJ, so
 	// you can use a scrolling floor to move scenery items underneath a bridge.
-	if ((tm.thing->flags2 & MF2_PASSMOBJ || thing->flags4 & MF4_ACTLIKEBRIDGE) && !(i_compatflags & COMPATF_NO_PASSMOBJ))
+	if ((tm.thing->flags2 & MF2_PASSMOBJ || thing->flags4 & MF4_ACTLIKEBRIDGE) && !(tm.thing->Level->i_compatflags & COMPATF_NO_PASSMOBJ))
 	{ // check if a mobj passed over/under another object
 		if (!(tm.thing->flags & MF_MISSILE) ||
 			!(tm.thing->flags2 & MF2_RIP) ||
@@ -1441,7 +1441,7 @@ bool PIT_CheckThing(FMultiBlockThingsIterator &it, FMultiBlockThingsIterator::Ch
 		{
 			clipheight = thing->projectilepassheight;
 		}
-		else if (thing->projectilepassheight < 0 && (i_compatflags & COMPATF_MISSILECLIP))
+		else if (thing->projectilepassheight < 0 && (thing->Level->i_compatflags & COMPATF_MISSILECLIP))
 		{
 			clipheight = -thing->projectilepassheight;
 		}
@@ -1763,7 +1763,7 @@ bool P_CheckPosition(AActor *thing, const DVector2 &pos, FCheckPosition &tm, boo
 			AActor *BlockingMobj = thing->BlockingMobj;
 
 			// If this blocks through a restricted line portal, it will always completely block.
-			if (BlockingMobj == NULL || (i_compatflags & COMPATF_NO_PASSMOBJ) || (tcres.portalflags & FFCF_RESTRICTEDPORTAL))
+			if (BlockingMobj == NULL || (thing->Level->i_compatflags & COMPATF_NO_PASSMOBJ) || (tcres.portalflags & FFCF_RESTRICTEDPORTAL))
 			{ // Thing slammed into something; don't let it move now.
 				thing->Height = realHeight;
 				return false;
@@ -2068,7 +2068,7 @@ static void CheckForPushSpecial(line_t *line, int side, AActor *mobj, DVector2 *
 {
 	if (line->special && !(mobj->flags6 & MF6_NOTRIGGER))
 	{
-		if (posforwindowcheck && !(i_compatflags2 & COMPATF2_PUSHWINDOW) && line->backsector != NULL)
+		if (posforwindowcheck && !(mobj->Level->i_compatflags2 & COMPATF2_PUSHWINDOW) && line->backsector != NULL)
 		{ // Make sure this line actually blocks us and is not a window
 			// or similar construct we are standing inside of.
 			DVector3 pos = mobj->PosRelative(line);
@@ -2175,7 +2175,7 @@ bool P_TryMove(AActor *thing, const DVector2 &pos,
 				goto pushline;
 			}
 		}
-		if (!(tm.thing->flags2 & MF2_PASSMOBJ) || (i_compatflags & COMPATF_NO_PASSMOBJ))
+		if (!(tm.thing->flags2 & MF2_PASSMOBJ) || (tm.thing->Level->i_compatflags & COMPATF_NO_PASSMOBJ))
 		{
 			thing->SetZ(oldz);
 			thing->flags6 &= ~MF6_INTRYMOVE;
@@ -2274,7 +2274,7 @@ bool P_TryMove(AActor *thing, const DVector2 &pos,
 
 		// compatibility check: Doom originally did not allow monsters to cross dropoffs at all.
 		// If the compatibility flag is on, only allow this when the velocity comes from a scroller
-		if ((i_compatflags & COMPATF_CROSSDROPOFF) && !(thing->flags4 & MF4_SCROLLMOVE))
+		if ((thing->Level->i_compatflags & COMPATF_CROSSDROPOFF) && !(thing->flags4 & MF4_SCROLLMOVE))
 		{
 			dropoff = false;
 		}
@@ -2963,7 +2963,7 @@ void FSlide::SlideTraverse(const DVector2 &start, const DVector2 &end)
 			goto isblocking;
 		}
 		if (li->flags & ML_BLOCKMONSTERS && !((slidemo->flags3 & MF3_NOBLOCKMONST)
-			|| ((i_compatflags & COMPATF_NOBLOCKFRIENDS) && (slidemo->flags & MF_FRIENDLY))))
+			|| ((slidemo->Level->i_compatflags & COMPATF_NOBLOCKFRIENDS) && (slidemo->flags & MF_FRIENDLY))))
 		{
 			goto isblocking;
 		}
@@ -4588,7 +4588,7 @@ AActor *P_LineAttack(AActor *t1, DAngle angle, double distance,
 			// Hit a thing, so it could be either a puff or blood
 			DVector3 bleedpos = trace.HitPos;
 			// position a bit closer for puffs/blood if using compatibility mode.
-			if (i_compatflags & COMPATF_HITSCAN)
+			if (trace.Actor->Level->i_compatflags & COMPATF_HITSCAN)
 			{
 				DVector2 ofs = t1->Level->GetPortalOffsetPosition(bleedpos.X, bleedpos.Y, -10 * trace.HitVector.X, -10 * trace.HitVector.Y);
 				bleedpos.X = ofs.X;
@@ -4781,7 +4781,7 @@ int P_LineTrace(AActor *t1, DAngle angle, double distance,
 	if ( flags & TRF_BLOCKSELF )
 	{
 		bool Projectile = ( (t1->flags&MF_MISSILE) || (t1->BounceFlags&BOUNCE_MBF) );
-		bool NotBlocked = ( (t1->flags3&MF3_NOBLOCKMONST) || ( (i_compatflags&COMPATF_NOBLOCKFRIENDS) && (t1->flags&MF_FRIENDLY) ) );
+		bool NotBlocked = ( (t1->flags3&MF3_NOBLOCKMONST) || ( (t1->Level->i_compatflags&COMPATF_NOBLOCKFRIENDS) && (t1->flags&MF_FRIENDLY) ) );
 		if ( Projectile ) lflags |= ML_BLOCKPROJECTILE;
 		if ( !Projectile || (t1->flags8&MF8_BLOCKASPLAYER) ) lflags |= ML_BLOCKING;
 		if ( !NotBlocked ) lflags |= ML_BLOCKMONSTERS;
@@ -5099,7 +5099,7 @@ static ETraceStatus ProcessRailHit(FTraceResults &res, void *userdata)
 	newhit.HitActor = res.Actor;
 	newhit.HitPos = res.HitPos;
 	newhit.HitAngle = res.SrcAngleFromTarget;
-	if (i_compatflags & COMPATF_HITSCAN)
+	if (res.Actor->Level->i_compatflags & COMPATF_HITSCAN)
 	{
 		DVector2 ofs = res.Actor->Level->GetPortalOffsetPosition(newhit.HitPos.X, newhit.HitPos.Y, -10 * res.HitVector.X, -10 * res.HitVector.Y);
 		newhit.HitPos.X = ofs.X;
@@ -5427,7 +5427,7 @@ bool P_UseTraverse(AActor *usething, const DVector2 &start, const DVector2 &end,
 				P_LineOpening(open, NULL, in->d.line, it.InterceptPoint(in));
 			}
 			if (open.range <= 0 ||
-				(in->d.line->special != 0 && (i_compatflags & COMPATF_USEBLOCKING)))
+				(in->d.line->special != 0 && (usething->Level->i_compatflags & COMPATF_USEBLOCKING)))
 			{
 				// [RH] Give sector a chance to intercept the use
 
@@ -5487,7 +5487,7 @@ bool P_UseTraverse(AActor *usething, const DVector2 &start, const DVector2 &end,
 			//[RH] And now I've changed it again. If the line is of type
 			//	   SPAC_USE, then it eats the use. Everything else passes
 			//	   it through, including SPAC_USETHROUGH.
-			if (i_compatflags & COMPATF_USEBLOCKING)
+			if (usething->Level->i_compatflags & COMPATF_USEBLOCKING)
 			{
 				if (in->d.line->activation & SPAC_UseThrough) continue;
 				else return true;
diff --git a/src/p_maputl.cpp b/src/p_maputl.cpp
index 4ea687216..16f6a72ad 100644
--- a/src/p_maputl.cpp
+++ b/src/p_maputl.cpp
@@ -1543,7 +1543,7 @@ void FPathTraverse::init(double x1, double y1, double x2, double y2, int flags,
 	// Count is present to prevent a round off error
 	// from skipping the break statement.
 
-	bool compatible = (flags & PT_COMPATIBLE) && (i_compatflags & COMPATF_HITSCAN);
+	bool compatible = (flags & PT_COMPATIBLE) && (Level->i_compatflags & COMPATF_HITSCAN);
 		
 	// we want to use one list of checked actors for the entire operation
 	FBlockThingsIterator btit(Level);
diff --git a/src/p_maputl.h b/src/p_maputl.h
index e22c9cddd..99a8a8a04 100644
--- a/src/p_maputl.h
+++ b/src/p_maputl.h
@@ -4,6 +4,7 @@
 #include <float.h>
 #include "r_defs.h"
 #include "doomstat.h"
+#include "doomdata.h"
 #include "m_bbox.h"
 
 extern int validcount;
@@ -50,7 +51,7 @@ inline int P_PointOnLineSidePrecise(const DVector2 &pt, const line_t *line)
 inline int P_PointOnLineSide (double x, double y, const line_t *line)
 {
 	extern int P_VanillaPointOnLineSide(double x, double y, const line_t* line);
-	return i_compatflags2 & COMPATF2_POINTONLINE
+	return (line->flags & ML_COMPATSIDE)
 		? P_VanillaPointOnLineSide(x, y, line) : P_PointOnLineSidePrecise(x, y, line);
 }
 
diff --git a/src/p_mobj.cpp b/src/p_mobj.cpp
index 52e413278..57216d5e3 100644
--- a/src/p_mobj.cpp
+++ b/src/p_mobj.cpp
@@ -1161,7 +1161,7 @@ bool AActor::Grind(bool items)
 		// see rh_log entry for February 21, 1999. Don't know if it is still relevant.
 		if (state == NULL 									// Only use the default crushed state if:
 			&& !(flags & MF_NOBLOOD)						// 1. the monster bleeeds,
-			&& (i_compatflags & COMPATF_CORPSEGIBS)			// 2. the compat setting is on,
+			&& (Level->i_compatflags & COMPATF_CORPSEGIBS)			// 2. the compat setting is on,
 			&& player == NULL)								// 3. and the thing isn't a player.
 		{
 			isgeneric = true;
@@ -1816,7 +1816,7 @@ double P_XYMovement (AActor *mo, DVector2 scroll)
 	// [anon] When friction is turned off, turn off the crouching and water
 	//  speed caps as well, since it is a sort of friction, and the modders
 	//  most likely want to deal with that themselves.
-	if ((mo->player != NULL && (i_compatflags & COMPATF_WALLRUN)) || ((mo->waterlevel >= 1 ||
+	if ((mo->player != NULL && (mo->Level->i_compatflags & COMPATF_WALLRUN)) || ((mo->waterlevel >= 1 ||
 		(mo->player != NULL && mo->player->crouchfactor < 0.75)) && !(mo->flags8 & MF8_NOFRICTION)))
 	{
 		// preserve the direction instead of clamping x and y independently.
@@ -1938,7 +1938,7 @@ double P_XYMovement (AActor *mo, DVector2 scroll)
 	DAngle oldangle = mo->Angles.Yaw;
 	do
 	{
-		if (i_compatflags & COMPATF_WALLRUN) pushtime++;
+		if (mo->Level->i_compatflags & COMPATF_WALLRUN) pushtime++;
 		tm.PushTime = pushtime;
 
 		ptry = start + move * step / steps;
@@ -1989,7 +1989,7 @@ double P_XYMovement (AActor *mo, DVector2 scroll)
 					// actor's velocity, do not attempt to slide.
 					if (mo->Vel.XY() == startvel)
 					{
-						if (player && (i_compatflags & COMPATF_WALLRUN))
+						if (player && (mo->Level->i_compatflags & COMPATF_WALLRUN))
 						{
 						// [RH] Here is the key to wall running: The move is clipped using its full speed.
 						// If the move is done a second time (because it was too fast for one move), it
@@ -2007,7 +2007,7 @@ double P_XYMovement (AActor *mo, DVector2 scroll)
 						}
 						else
 						{
-							if (!player || !(i_compatflags & COMPATF_WALLRUN))
+							if (!player || !(mo->Level->i_compatflags & COMPATF_WALLRUN))
 							{
 								move = mo->Vel;
 								onestep = move / steps;
@@ -2451,7 +2451,7 @@ void P_ZMovement (AActor *mo, double oldfloorz)
 	// Hexen yanked all items to the floor, except those being spawned at map start in the air.
 	// Those were kept at their original height.
 	// Do this only if the item was actually spawned by the map above ground to avoid problems.
-	if (mo->specialf1 > 0 && (mo->flags2 & MF2_FLOATBOB) && (ib_compatflags & BCOMPATF_FLOATBOB))
+	if (mo->specialf1 > 0 && (mo->flags2 & MF2_FLOATBOB) && (mo->Level->ib_compatflags & BCOMPATF_FLOATBOB))
 	{
 		mo->SetZ(mo->floorz + mo->specialf1);
 	}
@@ -3713,7 +3713,7 @@ void AActor::Tick ()
 						scrolltype <= Scroll_SouthWest_Fast)
 					{ // Hexen scroll special
 						scrolltype -= Scroll_North_Slow;
-						if (i_compatflags&COMPATF_RAVENSCROLL)
+						if (Level->i_compatflags&COMPATF_RAVENSCROLL)
 						{
 							scrollv.X -= HexenCompatSpeeds[HexenScrollies[scrolltype][0]+4] * (1. / (32 * CARRYFACTOR));
 							scrollv.Y += HexenCompatSpeeds[HexenScrollies[scrolltype][1]+4] * (1. / (32 * CARRYFACTOR));
@@ -3732,7 +3732,7 @@ void AActor::Tick ()
 						scrolltype -= Carry_East5;
 						uint8_t dir = HereticScrollDirs[scrolltype / 5];
 						double carryspeed = HereticSpeedMuls[scrolltype % 5] * (1. / (32 * CARRYFACTOR));
-						if (scrolltype < 5 && !(i_compatflags&COMPATF_RAVENSCROLL)) 
+						if (scrolltype < 5 && !(Level->i_compatflags&COMPATF_RAVENSCROLL))
 						{
 							// Use speeds that actually match the scrolling textures!
 							carryspeed = (1 << ((scrolltype % 5) + 15)) / 65536.;
@@ -3742,7 +3742,7 @@ void AActor::Tick ()
 					}
 					else if (scrolltype == dScroll_EastLavaDamage)
 					{ // Special Heretic scroll special
-						if (i_compatflags&COMPATF_RAVENSCROLL)
+						if (Level->i_compatflags&COMPATF_RAVENSCROLL)
 						{
 							scrollv.X += 28. / (32*CARRYFACTOR);
 						}
@@ -3795,7 +3795,7 @@ void AActor::Tick ()
 			// Some levels designed with Boom in mind actually want things to accelerate
 			// at neighboring scrolling sector boundaries. But it is only important for
 			// non-player objects.
-			if (player != NULL || !(i_compatflags & COMPATF_BOOMSCROLL))
+			if (player != NULL || !(Level->i_compatflags & COMPATF_BOOMSCROLL))
 			{
 				if (countx > 1)
 				{
@@ -3888,7 +3888,7 @@ void AActor::Tick ()
 		}
 		if (Vel.Z != 0 || BlockingMobj || Z() != floorz)
 		{	// Handle Z velocity and gravity
-			if (((flags2 & MF2_PASSMOBJ) || (flags & MF_SPECIAL)) && !(i_compatflags & COMPATF_NO_PASSMOBJ))
+			if (((flags2 & MF2_PASSMOBJ) || (flags & MF_SPECIAL)) && !(Level->i_compatflags & COMPATF_NO_PASSMOBJ))
 			{
 				if (!(onmo = P_CheckOnmobj (this)))
 				{
@@ -5414,7 +5414,7 @@ AActor *FLevelLocals::SpawnMapThing (FMapThing *mthing, int position)
 	if (sz == ONFLOORZ)
 	{
 		mobj->AddZ(mthing->pos.Z);
-		if ((mobj->flags2 & MF2_FLOATBOB) && (ib_compatflags & BCOMPATF_FLOATBOB))
+		if ((mobj->flags2 & MF2_FLOATBOB) && (mobj->Level->ib_compatflags & BCOMPATF_FLOATBOB))
 		{
 			mobj->specialf1 = mthing->pos.Z;
 		}
@@ -5437,7 +5437,7 @@ AActor *FLevelLocals::SpawnMapThing (FMapThing *mthing, int position)
 	}
 
 	// For Hexen floatbob 'compatibility' we do not really want to alter the floorz.
-	if (mobj->specialf1 == 0 || !(mobj->flags2 & MF2_FLOATBOB) || !(ib_compatflags & BCOMPATF_FLOATBOB))
+	if (mobj->specialf1 == 0 || !(mobj->flags2 & MF2_FLOATBOB) || !(mobj->Level->ib_compatflags & BCOMPATF_FLOATBOB))
 	{
 		P_FindFloorCeiling(mobj, FFCF_SAMESECTOR | FFCF_ONLY3DFLOORS | FFCF_3DRESTRICT);
 	}
diff --git a/src/p_pusher.cpp b/src/p_pusher.cpp
index c06ee2c1f..2f580048c 100644
--- a/src/p_pusher.cpp
+++ b/src/p_pusher.cpp
@@ -199,7 +199,7 @@ void DPusher::Tick ()
 			bool pusharound = ((thing->flags2 & MF2_WINDTHRUST) && !(thing->flags & MF_NOCLIP));
 					
 			// MBF allows any sentient or shootable thing to be affected, but players with a fly cheat aren't.
-			if (compatflags & COMPATF_MBFMONSTERMOVE)
+			if (Level->i_compatflags & COMPATF_MBFMONSTERMOVE)
 			{
 				pusharound = ((pusharound || (thing->IsSentient()) || (thing->flags & MF_SHOOTABLE)) // Add categories here
 					&& (!(thing->player && (thing->flags & (MF_NOGRAVITY))))); // Exclude flying players here
diff --git a/src/p_saveg.cpp b/src/p_saveg.cpp
index cb7cdc848..333992601 100644
--- a/src/p_saveg.cpp
+++ b/src/p_saveg.cpp
@@ -984,7 +984,9 @@ void FLevelLocals::Serialize(FSerializer &arc, bool hubload)
 
 	Behaviors.SerializeModuleStates(arc);
 	// The order here is important: First world state, then portal state, then thinkers, and last polyobjects.
+	SetCompatLineOnSide(false);	// This flag should not be saved. It solely depends on current compatibility state.
 	arc("linedefs", lines, loadlines);
+	SetCompatLineOnSide(true);
 	arc("sidedefs", sides, loadsides);
 	arc("sectors", sectors, loadsectors);
 	arc("zones", Zones);
diff --git a/src/p_sectors.cpp b/src/p_sectors.cpp
index 1694652d5..b9e17dd5e 100644
--- a/src/p_sectors.cpp
+++ b/src/p_sectors.cpp
@@ -473,9 +473,9 @@ double FindHighestCeilingSurrounding (const sector_t *sector, vertex_t **v)
 // jff 02/03/98 Add routine to find shortest lower texture
 //
 
-static inline void CheckShortestTex (FTextureID texnum, double &minsize)
+static inline void CheckShortestTex (FLevelLocals *Level, FTextureID texnum, double &minsize)
 {
-	if (texnum.isValid() || (texnum.isNull() && (i_compatflags & COMPATF_SHORTTEX)))
+	if (texnum.isValid() || (texnum.isNull() && (Level->i_compatflags & COMPATF_SHORTTEX)))
 	{
 		FTexture *tex = TexMan.GetTexture(texnum);
 		if (tex != NULL)
@@ -497,8 +497,8 @@ double FindShortestTextureAround (sector_t *sec)
 	{
 		if (check->flags & ML_TWOSIDED)
 		{
-			CheckShortestTex (check->sidedef[0]->GetTexture(side_t::bottom), minsize);
-			CheckShortestTex (check->sidedef[1]->GetTexture(side_t::bottom), minsize);
+			CheckShortestTex (sec->Level, check->sidedef[0]->GetTexture(side_t::bottom), minsize);
+			CheckShortestTex (sec->Level, check->sidedef[1]->GetTexture(side_t::bottom), minsize);
 		}
 	}
 	return minsize < FLT_MAX ? minsize : TexMan.ByIndex(0)->GetDisplayHeight();
@@ -522,8 +522,8 @@ double FindShortestUpperAround (sector_t *sec)
 	{
 		if (check->flags & ML_TWOSIDED)
 		{
-			CheckShortestTex (check->sidedef[0]->GetTexture(side_t::top), minsize);
-			CheckShortestTex (check->sidedef[1]->GetTexture(side_t::top), minsize);
+			CheckShortestTex (sec->Level, check->sidedef[0]->GetTexture(side_t::top), minsize);
+			CheckShortestTex (sec->Level, check->sidedef[1]->GetTexture(side_t::top), minsize);
 		}
 	}
 	return minsize < FLT_MAX ? minsize : TexMan.ByIndex(0)->GetDisplayHeight();
diff --git a/src/p_setup.cpp b/src/p_setup.cpp
index 4a9955dd9..602adff56 100644
--- a/src/p_setup.cpp
+++ b/src/p_setup.cpp
@@ -546,6 +546,7 @@ void P_SetupLevel(FLevelLocals *Level, int position, bool newGame)
 
 	Level->automap = AM_Create(Level);
 	Level->automap->LevelInit();
+	Level->SetCompatLineOnSide(true);
 
 	// [RH] Start lightning, if MAPINFO tells us to
 	if (Level->flags & LEVEL_STARTLIGHTNING)
diff --git a/src/p_sight.cpp b/src/p_sight.cpp
index 2e31e5f19..610025301 100644
--- a/src/p_sight.cpp
+++ b/src/p_sight.cpp
@@ -222,7 +222,7 @@ bool SightCheck::PTR_SightTraverse (intercept_t *in)
 //
 
 	// ignore self referencing sectors if COMPAT_TRACE is on
-	if ((i_compatflags & COMPATF_TRACE) && li->frontsector == li->backsector)
+	if ((Level->i_compatflags & COMPATF_TRACE) && li->frontsector == li->backsector)
 		return true;
 
 	double trX = Trace.x + Trace.dx * in->frac;
diff --git a/src/p_trace.cpp b/src/p_trace.cpp
index 624f11bd7..dd9c7bb99 100644
--- a/src/p_trace.cpp
+++ b/src/p_trace.cpp
@@ -421,7 +421,7 @@ bool FTraceInfo::LineCheck(intercept_t *in, double dist, DVector3 hit, bool spec
 
 		// For backwards compatibility: Ignore lines with the same sector on both sides.
 		// This is the way Doom.exe did it and some WADs (e.g. Alien Vendetta MAP15) need it.
-		if (i_compatflags & COMPATF_TRACE && in->d.line->backsector == in->d.line->frontsector && !special3dpass)
+		if (Level->i_compatflags & COMPATF_TRACE && in->d.line->backsector == in->d.line->frontsector && !special3dpass)
 		{
 			// We must check special activation here because the code below is never reached.
 			if (TraceFlags & TRACE_PCross)
diff --git a/src/po_man.cpp b/src/po_man.cpp
index 8c2aa750f..5147d77ff 100644
--- a/src/po_man.cpp
+++ b/src/po_man.cpp
@@ -1674,7 +1674,7 @@ void FPolyObj::CreateSubsectorLinks()
 		seg->v2 = side->V2();
 		seg->wall = side;
 	}
-	if (!(i_compatflags & COMPATF_POLYOBJ))
+	if (!(Level->i_compatflags & COMPATF_POLYOBJ))
 	{
 		SplitPoly(node, Level->HeadNode(), dummybbox);
 	}
diff --git a/src/s_sound.cpp b/src/s_sound.cpp
index 7179b8bbe..fd609cabe 100644
--- a/src/s_sound.cpp
+++ b/src/s_sound.cpp
@@ -848,7 +848,7 @@ static bool ValidatePosVel(const FSoundChan *const chan, const FVector3 &pos, co
 
 static void CalcSectorSoundOrg(const DVector3 &listenpos, const sector_t *sec, int channum, FVector3 &pos)
 {
-	if (!(i_compatflags & COMPATF_SECTORSOUNDS))
+	if (!(sec->Level->i_compatflags & COMPATF_SECTORSOUNDS))
 	{
 		// Are we inside the sector? If yes, the closest point is the one we're on.
 		if (currentUILevel->PointInSector(listenpos.X, listenpos.Y) == sec)
@@ -1663,7 +1663,7 @@ void S_StopSound (int channel)
 	{
 		FSoundChan *next = chan->NextChan;
 		if (chan->SourceType == SOURCE_None &&
-			(chan->EntChannel == channel || (i_compatflags & COMPATF_MAGICSILENCE)))
+			(chan->EntChannel == channel || (compatflags & COMPATF_MAGICSILENCE)))
 		{
 			S_StopChannel(chan);
 		}
@@ -1687,7 +1687,7 @@ void S_StopSound (AActor *actor, int channel)
 		FSoundChan *next = chan->NextChan;
 		if (chan->SourceType == SOURCE_Actor &&
 			chan->Actor == actor &&
-			(chan->EntChannel == channel || (i_compatflags & COMPATF_MAGICSILENCE)))
+			(chan->EntChannel == channel || (compatflags & COMPATF_MAGICSILENCE)))
 		{
 			S_StopChannel(chan);
 		}
@@ -1711,7 +1711,7 @@ void S_StopSound (const sector_t *sec, int channel)
 		FSoundChan *next = chan->NextChan;
 		if (chan->SourceType == SOURCE_Sector &&
 			chan->Sector == sec &&
-			(chan->EntChannel == channel || (i_compatflags & COMPATF_MAGICSILENCE)))
+			(chan->EntChannel == channel || (compatflags & COMPATF_MAGICSILENCE)))
 		{
 			S_StopChannel(chan);
 		}
@@ -1735,7 +1735,7 @@ void S_StopSound (const FPolyObj *poly, int channel)
 		FSoundChan *next = chan->NextChan;
 		if (chan->SourceType == SOURCE_Polyobj &&
 			chan->Poly == poly &&
-			(chan->EntChannel == channel || (i_compatflags & COMPATF_MAGICSILENCE)))
+			(chan->EntChannel == channel || (compatflags & COMPATF_MAGICSILENCE)))
 		{
 			S_StopChannel(chan);
 		}
@@ -1827,7 +1827,7 @@ void S_ChangeSoundVolume(AActor *actor, int channel, double dvolume)
 	{
 		if (chan->SourceType == SOURCE_Actor &&
 			chan->Actor == actor &&
-			(chan->EntChannel == channel || (i_compatflags & COMPATF_MAGICSILENCE)))
+			(chan->EntChannel == channel || (compatflags & COMPATF_MAGICSILENCE)))
 		{
 			GSnd->ChannelVolume(chan, volume);
 			chan->Volume = volume;
@@ -1933,7 +1933,7 @@ static bool S_IsChannelUsed(AActor *actor, int channel, int *seen)
 
 bool S_IsActorPlayingSomething (AActor *actor, int channel, int sound_id)
 {
-	if (i_compatflags & COMPATF_MAGICSILENCE)
+	if (compatflags & COMPATF_MAGICSILENCE)
 	{
 		channel = 0;
 	}
diff --git a/src/scripting/backend/codegen.cpp b/src/scripting/backend/codegen.cpp
index 2a854d346..6b5a4b9f8 100644
--- a/src/scripting/backend/codegen.cpp
+++ b/src/scripting/backend/codegen.cpp
@@ -6858,9 +6858,7 @@ ExpEmit FxCVar::Emit(VMFunctionBuilder *build)
 		int *pVal;
 		auto cv = static_cast<FFlagCVar *>(CVar);
 		auto vcv = &cv->ValueVar;
-		if (vcv == &compatflags) pVal = &i_compatflags;
-		else if (vcv == &compatflags2) pVal = &i_compatflags2;
-		else pVal = &vcv->Value;
+		pVal = &vcv->Value;
 		build->Emit(OP_LKP, addr.RegNum, build->GetConstantAddress(pVal));
 		build->Emit(OP_LW, dest.RegNum, addr.RegNum, nul);
 		build->Emit(OP_SRL_RI, dest.RegNum, dest.RegNum, cv->BitNum);
diff --git a/src/scripting/vmthunks_actors.cpp b/src/scripting/vmthunks_actors.cpp
index a670907bb..d4b8f260a 100644
--- a/src/scripting/vmthunks_actors.cpp
+++ b/src/scripting/vmthunks_actors.cpp
@@ -1666,6 +1666,34 @@ DEFINE_ACTION_FUNCTION_NATIVE(AActor, isFrozen, isFrozen)
 }
 
 
+//=====================================================================================
+//
+// compat flags. These two are the only ones that get checked in script code
+// so anything more complex isn't really needed.
+//
+//=====================================================================================
+static int compat_limitpain_(AActor *self)
+{
+	return self->Level->i_compatflags & COMPATF_LIMITPAIN;
+}
+
+static int compat_mushroom_(AActor *self)
+{
+	return self->Level->i_compatflags & COMPATF_MUSHROOM;
+}
+
+DEFINE_ACTION_FUNCTION_NATIVE(AActor, compat_limitpain, compat_limitpain_)
+{
+	PARAM_SELF_PROLOGUE(AActor);
+	ACTION_RETURN_INT(compat_limitpain_(self));
+}
+
+DEFINE_ACTION_FUNCTION_NATIVE(AActor, compat_mushroom, compat_mushroom_)
+{
+	PARAM_SELF_PROLOGUE(AActor);
+	ACTION_RETURN_INT(compat_mushroom_(self));
+}
+
 //===========================================================================
 //
 // PlayerPawn functions
diff --git a/src/swrenderer/line/r_line.cpp b/src/swrenderer/line/r_line.cpp
index 731292147..2b3578437 100644
--- a/src/swrenderer/line/r_line.cpp
+++ b/src/swrenderer/line/r_line.cpp
@@ -591,7 +591,7 @@ namespace swrenderer
 			if (mFrontSector->e && mFrontSector->e->XFloor.lightlist.Size()) return true;
 			if (mBackSector->e && mBackSector->e->XFloor.lightlist.Size()) return true;
 
-			if (sidedef->GetTexture(side_t::mid).isValid() && ((ib_compatflags & BCOMPATF_CLIPMIDTEX) || (linedef->flags & (ML_CLIP_MIDTEX | ML_WRAP_MIDTEX)) || sidedef->Flags & (WALLF_CLIP_MIDTEX | WALLF_WRAP_MIDTEX))) return true;
+			if (sidedef->GetTexture(side_t::mid).isValid() && ((mFrontSector->Level->ib_compatflags & BCOMPATF_CLIPMIDTEX) || (linedef->flags & (ML_CLIP_MIDTEX | ML_WRAP_MIDTEX)) || sidedef->Flags & (WALLF_CLIP_MIDTEX | WALLF_WRAP_MIDTEX))) return true;
 
 			return false;
 		}
@@ -653,7 +653,7 @@ namespace swrenderer
 
 				if (sidedef->GetTexture(side_t::mid).isValid())
 				{
-					if (ib_compatflags & BCOMPATF_CLIPMIDTEX) return true;
+					if (mFrontSector->Level->ib_compatflags & BCOMPATF_CLIPMIDTEX) return true;
 					if (linedef->flags & (ML_CLIP_MIDTEX | ML_WRAP_MIDTEX)) return true;
 					if (sidedef->Flags & (WALLF_CLIP_MIDTEX | WALLF_WRAP_MIDTEX)) return true;
 				}
diff --git a/src/swrenderer/line/r_renderdrawsegment.cpp b/src/swrenderer/line/r_renderdrawsegment.cpp
index 0f477d70c..610641fdf 100644
--- a/src/swrenderer/line/r_renderdrawsegment.cpp
+++ b/src/swrenderer/line/r_renderdrawsegment.cpp
@@ -150,7 +150,7 @@ namespace swrenderer
 			return false;
 
 		FTexture *ttex = TexMan.GetPalettedTexture(curline->sidedef->GetTexture(side_t::mid), true);
-		if (i_compatflags & COMPATF_MASKEDMIDTEX)
+		if (curline->GetLevel()->i_compatflags & COMPATF_MASKEDMIDTEX)
 		{
 			ttex = ttex->GetRawTexture();
 		}
@@ -294,7 +294,7 @@ namespace swrenderer
 			  // this drawseg.
 				if ((curline->linedef->flags & ML_CLIP_MIDTEX) ||
 					(curline->sidedef->Flags & WALLF_CLIP_MIDTEX) ||
-					(ib_compatflags & BCOMPATF_CLIPMIDTEX))
+					(curline->GetLevel()->ib_compatflags & BCOMPATF_CLIPMIDTEX))
 				{
 					ClipMidtex(x1, x2);
 				}
@@ -356,7 +356,7 @@ namespace swrenderer
 			  // this drawseg.
 				if ((curline->linedef->flags & ML_CLIP_MIDTEX) ||
 					(curline->sidedef->Flags & WALLF_CLIP_MIDTEX) ||
-					(ib_compatflags & BCOMPATF_CLIPMIDTEX))
+					(curline->GetLevel()->ib_compatflags & BCOMPATF_CLIPMIDTEX))
 				{
 					ClipMidtex(x1, x2);
 				}
diff --git a/src/swrenderer/things/r_visiblespritelist.cpp b/src/swrenderer/things/r_visiblespritelist.cpp
index 8c3935a4b..ca1a40ab5 100644
--- a/src/swrenderer/things/r_visiblespritelist.cpp
+++ b/src/swrenderer/things/r_visiblespritelist.cpp
@@ -69,7 +69,7 @@ namespace swrenderer
 		if (count == 0)
 			return;
 
-		if (!(i_compatflags & COMPATF_SPRITESORT))
+		if (!(thread->Viewport->Level()->i_compatflags & COMPATF_SPRITESORT))
 		{
 			for (unsigned int i = 0; i < count; i++)
 				SortedSprites[i] = Sprites[first + i];
diff --git a/wadsrc/static/zscript/doom/fatso.txt b/wadsrc/static/zscript/doom/fatso.txt
index f24b0d2e7..cbf820398 100644
--- a/wadsrc/static/zscript/doom/fatso.txt
+++ b/wadsrc/static/zscript/doom/fatso.txt
@@ -105,6 +105,7 @@ extend class Actor
 {
 	const FATSPREAD = 90./8;
 	
+	private native bool compat_mushroom();
 
 	void A_FatRaise()
 	{
@@ -195,7 +196,7 @@ extend class Actor
 		aimtarget.Height = Height;
 		
 		bool shootmode = ((flags & MSF_Classic) || // Flag explicitly set, or no flags and compat options
-					(flags == 0 && CurState.bDehacked && compat_mushroom));
+					(flags == 0 && CurState.bDehacked && compat_mushroom()));
 
 		for (i = -numspawns; i <= numspawns; i += 8)
 		{
diff --git a/wadsrc/static/zscript/doom/painelemental.txt b/wadsrc/static/zscript/doom/painelemental.txt
index f953e47ef..7995eae1b 100644
--- a/wadsrc/static/zscript/doom/painelemental.txt
+++ b/wadsrc/static/zscript/doom/painelemental.txt
@@ -62,6 +62,8 @@ class PainElemental : Actor
 
 extend class Actor
 {
+	private native bool compat_limitpain();
+
 	//
 	// A_PainShootSkull
 	// Spawn a lost soul and launch it at the target
@@ -86,7 +88,7 @@ extend class Actor
 		}
 
 		// [RH] make this optional
-		if (limit < 0 && compat_limitpain)
+		if (limit < 0 && compat_limitpain())
 			limit = 21;
 
 		if (limit > 0)