Like for instance, monsters and other sprite objects in the original incarnation of the Doom engine have infinite height. So you can't step on top of, or over, any monsters if e.g. you are on a ledge high above them. That's because they're 2D objects, and their vertical position on the screen is largely only cosmetic. This is why you can't run under a Cacodemon, for instance.

"Actors" (monsters, etc.) in Doom do have defined heights, but presumably for speed purposes the engine ignores this except for a small subset of checks, namely for projectile collision and checking whether a monster can enter a sector or if the ceiling height is too low, and for crush damage.

This was rectified in later versions of the Doom engine as well as most source ports. By the time Heretic came out (which is just chock-a-block full of flying enemies and also allows the player to fly with a powerup) monsters no longer had infinite height.