ravelin
A kind of outwork; a freestanding, wedge-shaped fortification used to split an attacking force as they approach a larger fortress.
It is about 2,500 yards in circuit, is built of red stone, and, according to Von Orlich, is now " a bastioned quinquangle ; the ancient walls with semicircular bastions face the two streams ; the land side is quite regular, and consists of two bastions, and a half-bastion with three ravelins," and stands higher than any ground in face of it.
Where it comes from
From French ravelin, from Italian rivellino, of unclear origin: perhaps from Latin re- + vallum.
Adapted from the Wiktionary entry ravelin (CC BY-SA 4.0).