RORDoom

From Doom Wiki

RORDoom was an early attempt by Julian Aubourg at implementing a room-over-room effect in Doom. Based on Boom, it was released shortly before EDGE, which provided a similar feature. EDGE gained in popularity, and RORDoom's authors stopped their effort.

In RORDoom, level designers could create sectors on top of other sectors, thus breaking the rule of non-overlapping areas. BSP tree pre-compilation was done as usual, and RORDoom's engine detected subsectors that belonged to several sectors at loading time.