Room-over-room

Room-over-room designates methods allowing to transcend Doom's 2.5D 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.) or through source port features, notably 3D middle textures, 3D floors and portals; with only the latter two offering a fully convincing effect.

One of the earliest implementation, provided by RORDoom, allowed sectors to overlap. This approach is not generally used now.

Doom 64 simulates a ROR effect by changing the floor and ceiling height depending from the direction the player is coming from. This effect is noticeable while using a no clipping cheat since the sectors won't change properly. Theorically multiplayer would break that effect, however Doom 64 has no support for multiplayer.