A ravelin (originally known as a demi-lune or half-moon) is a standalone, triangular fortification outwork positioned outside a castle or fortress, directly in front of the inner curtain walls and bastions. Emerging during the Renaissance with the advent of gunpowder warfare, it acted as a detached shield to absorb cannon fire and disrupt attacking ground forces.

Structural Features:
The Salient Angle: The outward-facing front forms an acute V-shape. This specific angle deflects incoming enemy artillery fire to the left or right, rather than absorbing a direct, devastating impact.

The Open Gorge (Rear): The side facing inward toward the main castle is left entirely open, or built with only a very low wall. If the enemy successfully overruns and captures the ravelin, they have no rear defensive shelter to hide behind, leaving them completely exposed to the primary castle walls.

Earthwork and Masonry Thickening: The outer walls are heavily sloped and typically backed by compressed, solid earth. This absorbs the energy of cannonballs, rather than letting them shatter the stonework.

Internal Casemates and Tunnels: Advanced ravelins house vaulted internal chambers and tunnels (casemates) equipped with gun loops. These allow musketeers and small artillery crews to fire safely from behind cover.

Defensive Functions:
Forces Tactical Splitting: The triangular shape naturally forces an advancing assault army to divide its troops into separate, weaker groups on either side of the point.

Provides Flanking Fire: Soldiers or cannons positioned atop the ravelin can unleash crossfire (flanking fire) directly onto enemy troops moving through the moat or ditch.

Protects Core Weak Points: Ravelins were systematically built to protect bridges, drawbridges, and main entrance gates from direct frontal battery assaults.

Delays Siege Lines: By forcing the enemy to conquer the outwork first, it effectively burns the attackers' time, ammunition, and manpower before they ever reach the main stronghold.