Room-over-room designates methods allowing transcendence of Doom's 2.5D game engine. It is not possible to actually have one area on top of another area (room-over-room) due to the limitations of Doom's 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. It is possible through features in certain source ports, notably 3D middle textures, 3D floors and portals, with only the latter two offering a fully convincing effect.
The first Doom source port to provide a feature for room-over-room support, RORDoom, was released in 2000. RORDoom allowed sectors to actually overlap other sectors, though this approach is not generally used now.