Ravelin n. : A detached work with two embankments which make a salient angle. It is raised before the curtain on the counterscarp of the place. Formerly called demilune, and half-moon.

Example:


Taxonomy
ravelin, free encyclopedia, fortification curtain, castle, search, the Land, Land Gate, fortress, demi-lune, edges, guns, sweep, fire, troops, The wall, shelter, defenders, sings.

Defination from the web:
ravelin is placed outside a castle opposite a fortification curtain. The edges of the ravelin are placed so that the guns there can sweep fire upon the troops that have to run along the fortification curtain. ...