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Room-over-room is a term used to describe placing a room directly above another, often used in reference to id Software's Doom series. In the original Doom games, all mapping was done in 2D, while height variance was done via numbers, so getting the effect to work was an achievement at the time[clarification needed An achievement for Doom modders who pulled it off, or an achievement for game developers who managed to implement room over room in their own games?]. In true 3D engines, such as the Quake engine, this is an easy effect to pull off.
The restriction was originally solved by using a silent transport to fake the effect, but has since been completely corrected in some source ports, such as Doom Legacy. In game engines such as Cube, room-over-room can be mimicked using 3D models.