Fortified tower

Square or rectangular towers are easy to construct and give a good amount of usable internal space.

Despite this vulnerability, rectangular towers continued to be used, and Muslim military architecture generally favoured them.

The semicircular side (the one facing the attacker) could resist siege engines, while the rectangular part at the back gives internal space and a large fighting platform on top.

A common form is an octagonal tower, used in some bergfrieds and at Castel del Monte in Italy.

For instance, the keep at Château Gaillard is slightly bent forward, but also has a triangular beak to deflect projectiles.