A hornwork is an element of the trace italienne system of fortification

Fortification

Fortifications are military constructions and buildings designed for defence in warfare and military bases. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs...

. It consists of a pair of demi-bastion

Bastion

A bastion, or a bulwark, is a structure projecting outward from the main enclosure of a fortification, situated in both corners of a straight wall , facilitating active defence against assaulting troops...

s with a curtain wall connecting them and with two long sides directed upon the faces of the bastions, or ravelin

Ravelin

A ravelin is a triangular fortification or detached outwork, located in front of the innerworks of a fortress...

s of the inner fortifications, so as to be defended by them.

The hornwork was used to extend the fortified area in a particular direction to prevent the enemy occupying an area of high ground or simply strengthen the overall fortifications in the expected direction of attack. A crownwork serves a similar purpose, but unlike a hornwork, it contains full bastion.