Submitting this separately since I don't believe this was ever properly displayed in Doomsday.
Map21 of TNT has doors where, with the use of render hacks, you can look through the doors into the rooms. You can find these around the perimeter of the starting area, all but one of them which require keys to open. In the 2.2 release of Doomsday, the doors are simply black, while in the current 2.3 unstable releases of Doomsday, nothing is rendered and you can completely see right into the room (the door will open, though).
In the second screenshot, it's at -1800, 600. There are supposed to be bars there in between you and the enemies, but nothing there is rendered.