Really cool, I also remember poking around in the internals of Duke Nukem 3D and making levels a lot.

One cool thing about die Build engine is portals, you can build two different rooms and identify a wall in one with another wall in the other and "glue" the rooms together, IIRC. I think that is what is behind most of the "teleportation". I always thought that the mirror was just a wall portaled to itself, I wonder why the "physical" mirror space is necessary?

Edit: or maybe I'm misremembering what portals did? Here is a really detailed article by Fabien Sanglard about the engine: https://fabiensanglard.net/duke3d/build_engine_internals.php