Ravelins

A ravelin is a projecting outwork in historic fortifications, typically triangular or wedge-shaped, built in front of a fortress's main curtain wall. It is usually detached from the principal ramparts and situated between two bastions, separated from the curtain by a ditch or moat.

The purpose of a ravelin is to disrupt direct approaches to the curtain, absorb or deflect artillery fire, and provide enfilading fire from the flanking bastions. By creating an intermediate obstacle, a ravelin protects the covered way and the face of the main walls, and complicates assaults by forcing attackers to contend with multiple angles and fire directions.

Typically, a ravelin has two faces that form a salient toward the field, with its own parapet, glacis, and ditch. It is commonly connected to the fortress by a bridge or covered way, or may be largely detached with only a sally port.

Historically, ravelins were a standard feature of late medieval and early modern European fortifications, especially in Vauban-style fortresses of the 17th and 18th centuries. Their use declined as artillery and siege tactics evolved, but many surviving forts retain ravelins as part of their complex defensive systems.

The term ravelin comes from French, and is sometimes spelled raveline.