It's surely both the sprite-based monsters and the "2.5D" world that justify DOOM's "2.5D/fake 3D" label, though indeed more the latter. (System Shock was a transitional example of sprite monsters in an otherwise full-3D world: https://youtu.be/CGKidgwcalM?list=PLqqLcEJ9bUtNedMdW64WioyV9... )
