Cones have the same sort of problem as spheres, you can't see past the "bulge". And the roofs on your example are not designed to have people fighting on them. Just to shed flaming projectiles and rain.

A box base is possible. I'm thinking a set of battlements at the top of the box (or an equivalent height up the sides of the pyramid. Complete with turrets (castle type, not gun type) at the corners.

Repeat as needed up the sides. and a lookout at the top.

Ventilation would important, but quite doable even without magic.

Sieges get interesting when you have purify food & water as well as *create* food and water. Cure disease will be a big help.