[ant6n]

Portals allow weird stuff (non linear geometry) in a BSP level. I thought Doom had petals.

[dfox]

The primary thing that BSP does is that it maps coordinates onto a BSP node/sector. Thus you cannot have overlapping geometry, as this mapping would not be unique. Quake has some idea of portals (I'm not sure about Doom), but it is used only as an additional layer of optimization, the engine is not fundamentally portal-based.

[jorvi]

There is a very interesting Doom level named “myhouse.wad” that does a lot of clever things to seemingly allow room-over-room.

[Narishma]

I don't think that mod works on a normal Doom engine.