A ravelin is a triangular fortification or detached outwork, located in front of the innerworks of a fortress (the curtain walls and bastions). Originally called a demi-lune, after the lunette, the ravelin is placed outside a castle and opposite a fortification curtain.

The ravelins of Animas and of the Rosary are covering adjacent curtains to the bastion of Santa Tecla, they are equal to each other, though of smaller dimensions than the previous ones and each has two domes and a cistern.

The outer edges of the ravelin are so configured that it divides an assault force and from the ravelin guns can fire upon the attacking troops as they approach the curtain. It also impedes besiegers from using their artillery to batter a breech in the curtain wall. The side of the ravelin facing the inner fortifications has at best a low wall, if any, so that the ravelin would not shelter attacking forces should they have overwhelmed the ravelin, or the defenders have abandoned it. Frequently ravelins have a ramp or stairs on the curtain-wall side to facilitate the movement of troops and artillery onto the ravelin.