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 s

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.