[Someone]

If you build a round tower or a circular fort (which is optimal in both area/wall ratio and strength) (edit: or use any other ‘locally convex’ layout), attackers who manage to sneak up to the wall at a location where there isn’t any opening in the wall (i.e. most places) can hack away at the wall relatively (they may still be hit from above, but even that may be limited, depending on the design of the wall) unimpeded, as nobody shooting projectiles out of the fort can hit them. So, such attackers could work for days or even weeks to bring down part of the wall.

These designs are such that, wherever attackers are standing near the wall, there’s a position inside the fort from where one can hit them, often from both sides.

You also don’t want you projectiles, aimed at attackers, to hit and damage your own wall.

Projectile range and rate of fire mean one cannot simple scale up a smaller design to make a larger one.