I remember fondly that I was toying around with the level editor of Duke Nukem 3D as a kid. Now I understand why it wouldn't really work out the way I assumed it would. Not only did I not understand much of english back then, but it was actually confined to 2.5D ...
There were a few other places in the first level that had "room above room" hacks that only work if you can't see overlapping areas of both rooms at the same time.
In the movie theatre there is a projector room up high, and the theatre exit below BUT the theatre exit has a wall centered in the middle where the overlap would be. There is also a spiral staircase going up to the projector room which has 2 or 3 levels of overlap, but as it is enclosed you don't view 2 levels at the same time.
Interesting stuff - and like many others have commented, I remember playing around with the Build editor and learning about these random hacks.
EDIT: and just now I'm remembering that submerging in water was another hack - you would actually teleport to another room on the map. In Duke3d the water was opaque, but in the later Shadow Warrior i remember it having some transparency (I assume probably using similar mirror hacks as D3d)
