Looking at the source for Descent would be rather fascinating - it was certainly one of the earliest examples of a 'true' 3D engine, with 3DoF, where DOOM was still locked to flat surfaces.
(ZDoom, and the beyond-incredible Sonic fan game SRB2, based on the DOOM II source, both support slopes in their engine, at this point).
I'm not gonna lie, Descent blew my mind when I was like 11 and fiddling with the DOOM II source.
The code for Descent has been available for a long time and if you're going to look, you should probably check out some of the links and info here: http://icculus.org/d2x/
Mike: ... At first all I cared about was writing technically good code.
Matt: Then we ran out of money and all we cared about was finishing our game.
Mike: Right. Our code got ugly, but our game got done.
[1] https://github.com/videogamepreservation/descent