The most common 'teleport' is simply intersecting rooms. Take a look at pentacircle: flying around gives you a whole bunch of different, but similarly stacked areas. Other's, such have duality, appear to have two identical but overlapped maps. Then there's the more recent antbend, which takes a while to even notice that there are intersecting rooms.
But this is not teleportation, it's simply that cubes can occupy the same space and not be connected - or even visible.
Teleportation is what could move you from one side of the map to the other side of the map: the simplest example is to have an infinitely long corridoor, where one end teleports you to the other. (Which would make a weird level in itself).
The reason this is possible is because Descent uses cubes as it's building block. In the level file, each cube contains information about:
- It's vertices
- It's textures
and more interestingly:
- What cubes it joins to via which edges. Quite literally you have a six-bit bitmask followed by up to six cube ID's.
(You can find the spec here: http://www.descent2.com/ddn/specs/rdl/
4D space is simple, you simply have cubes occupying the same space that don't join, and from one cube you won't see the other.
Teleporters could be done by manually changing the cube ID. If I have cube 10 on one side of the level, I could manually set it to join to cube 30 on the other side of the level using a hex editor. I will say though, the level file is a nightmare to go through. I once wrote an importer so that blender could open .rdl files, it was a nightmare. I'd probably be better now that I have a little more understanding of both the format and general computers.
Well, good luck....