In Duke3D's game.c file you'll find a function called animatesprites. In here there's a bunch of code for doing things like hiding parent-mode tiles etc, but also for doing the fake 3D sprites. If you look down around half way in the function there's a a block of code that begins with:
if( actorscrptr[s->picnum] )
{
if(t4)
{
This is the the code to set the proper viewpoint for the enemies in the game (stuff defined in the CONs with actor or useractor) and includes the logic to handle a variety of viewpoint counts, from two to eight.
Jonathon