Room-over-room

Room-over-room designates methods allowing to transcend Doom's 2.5D engine. While it is not possible to actually have one area on top of another area (room-over-room) due to the limitations of the game engine, the effect can be faked in the normal Doom rendering engine through various tricks (such as moving sectors, as in Industrial Zone's south-east building, self-referencing sectors, etc.) to give the illusion of having room-over-room, or through features in source ports, notably 3D middle textures, 3D floors and portals, with only the latter two offering a fully convincing effect.

One of the earliest implementations of room-over-room, provided by RORDoom, allowed sectors to overlap, though this approach is not generally used now.