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?
I'm fairly sure portals in that case only refers to how the Build engine determined potentially visible areas. As far as I know there was no mechanism to seamlessly connect far-away parts of a map in that way. If there was, I'd be pissed, because I never used that to built my own maps and it would allow neat non-euclidean geometry (like in Antichamber) :-)
Yeah, you are right. I think the engine I recall was the successor of Genesis3D, some ActiveX-thing from WildTangent. Whole different beast. I remember making a Stargate using that feature :-D. Must have been around 2000.
I was always got a kick of out of the swimming sections simply being teleporters. Step on top of the water a get teleported to a room filled with water and then swim to the top of the room to get teleported back to the area around the pool.
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