Neither of those games used cylindrical billboards, the basic idea is that both of those engines utilized raycasting to create the pseudo 3d environment and used traditional sprite rendering (scaled by the distance from the player) for the enemies. There are 8 directions for all enemy animations, and the engine compares the direction of an imaginary line from the camera's position to the enemy's position with the direction the enemy is facing, rounded to the nearest 45 degrees, to determine which sprite angle is shown.
I've been looking into this for a couple months because I also really like the look and feel of these games, but making all these images by hand is a lot of work. Once actual 3d rendering became the norm it saved artists a lot of time and sanity. Your best bet when making these kind of sprites is to use a 3d modeling program to at least block everything out for the directions and keyframes and then pixel over top of those. The Doom artist, Adrian Carmack,
used photos of clay models as bases for his sprites, which (despite sounding like a lot of fun) is probably only due to the technological limitations at the time.
Do you have any specific questions?
