Originally E1M2. This level was created back when there was no
BSP for DOOM's levels and Carmack was using a sectorlist for rendering.
This is the area that John Romero created that partially forced Carmack to look
for faster ways of stepping through the level
data for quick rendering. The problem with this area was the recursive sectorlist
stepping problem - the renderer goes into a sector, then
another sector, another, etc. until it's stepping into the same sectors all
over again: problems.
E1M2 prototype - down those stairs
Same E1M2 prototype
Same E1M2 prototype - down the hall