>>12016692
1. Projectiles are 2d sprites with a height property
2. Pillar is not a 3d object, it's an elevation of floor
The point is that it's more like Wolfenstein with smoke an mirrors. There's very little 3d math compared to full 3d engine like Quake. All the DOOM levels were made in a 2d map editor.
For example you could draw an image with 8 color palette but using dithering so it would appear that the image is more colorful. However, it is still an 8 color image.