Bastion

Bastion (old French bastir, to build), a projecting outwork of a fortress consisting of two flanks connected by two faces which meet at an acute angle (called the salient angle). Its object is to command all the ground immediately in front of the fortification, and bring artillery fire to bear on assailants. Detatched bastions, introduced by Vauban, are separated from the work they protect by a ditch.