RORDoom, created by Julian Aubourg and released in 2000, was the first source port to allow room-over-room support to be implemented in Doom. It was a DOS-based source port based on Boom and released shortly before EDGE, which provided a similar feature. As EDGE increased in popularity, development on RORDoom stopped.
In RORDoom, level designers could create sectors on top of other sectors, thus breaking the original Doom engine's limitation 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.