I remember this being addressed on the NewDoom forums, but it's probably buried in there now.
In Cyberden, you can see the Cyberdemons but they can't see you.
Surrounding them is a sector where the floor is raised to 1 unit below the ceiling.
This doesn't appear to work in Doomsday, as you see a texture, so you can't see the Cyberdemon and the Cyberdemon can't see you.
I remember DaniJ making a post on this on the other forums explaining it and it was something related to DE.
I'm telling ya, when DE 2.0 is out, it is going to be VERY good.