when talking about these old engines, you kind of have to remember that back then, we didn't have 3d acceleration, 3d apis (or any gaming apis. direct x didn't exist), or anything like that. it was all done through software rendering. this means that the only thing doing the processing for the rendering was the main cpu (386/486/etc). back then, we didn't even have graphics cards with 3d hardware on them. they were all made to just do 2d. even getting video modes like 640x480 was difficult until the VESA standard came about, and even THAT was a little flaky.
eventually we got some basic 3d cards out there, and quake had its first 3d accelerated version running on the verite (which was pretty nice), and then after that carmack rewrote the rendering to go through the opengl api. you also have to remember, back then we didn't even have hardware T&L! The software rendering in quake was way ahead of its time though. The lighting effects, and mip mapping through software was just genius, and if you ran it on a pentium, it was really, really fast (at 320x220).
There were some efforts to port the build engine to the Glide API (the one used by 3dfx), but as far as i know, that only happened with Blood. It ran alright, but was a little buggy, and was never really finished.
BTW, Blood enabled overlapping sectors to have holes in them, thus allowing you to look down from one sector into another. Build was awesome in that it even though it wasn't really 3d, it had a lot of tricks to make the player think it was.
Anyway, it's great that you guys are so interested in 3d game development, and here are a few topics you might want to read up on:
Software Rendering
Hardware Rendering
3d APIs (opengl, direct3d)
I find it amazing that people are STILL talking about these old games, over 10 years later
Don't know if there's any old guys like me on the forums here, but it's good to see the old code appreciated 