: bullets, including rocks, treat other enemies as pillars reaching from
: ground to ceiling. The size of the pillar need not match the size of the
: sprite the enemy is drawn with; in particular, you might see arms and legs
: sticking outside the pillar, if you could see the pillar. All monsters are
: the same size pillar.)
All good advice, but this is not 100% accurate. There's a set of flags associated with each monster and projectile that splits this pillar into low, middle, and high "zones" and lets projectiles pass through zones that are not occupied by the monster. This is how Greater Nightmares can shoot through Venomous Skitters and vice versa.
The vast majority of enemies have both the low and middle flags set (there's a "humanoid" macro for this) and the vast majority of projectiles are flagged as middle. Nightmares, GNs, Wraiths, and Spheres only use the middle flag. Skitters, Venomous Skitters, and Sentinels are flagged as Low. Only the boss monsters use the high flag; they occupy all 3 zones. The Phantasm has a special "transparent" flag which exempts it from all projectile impacts.