A postern gate is a small, secondary door, gate, or concealed passageway built into the defensive wall of a castle, fortification, or walled city. Traditionally located in the rear, flanks, or hidden recesses of a curtain wall, it served as a highly strategic, low-profile exit and entry point during sieges and daily operations.

Core Strategic Functions:
Surprise Counter-Attacks: Allowed defenders to slip out undetected to launch sudden ambushes (sorties) on besieging forces.

Secret Communications: Enabled messengers to leave or enter the castle secretly to request reinforcements or gather intelligence.

Emergency Escape Route: Provided a final, discrete escape path for royals, nobility, or garrison commanders if the main gates fell.

Peacetime Convenience: Served as a practical everyday entrance for trusted personnel, preventing the need to open the massive, vulnerable main gatehouses.

Key Architectural Features:
Concealed Locations: Often built into sharp angles of walls, behind towers, or high above the ground reachable only by removable wooden ladders.

Narrow Dimensions: Designed intentionally small—frequently only wide enough for a single horse or a few men on foot to pass at once.

Heavy Fortification: Heavily reinforced with iron-studded wooden doors, sturdy internal drawbars, and sometimes independent portcullises.

Defensive Chokepoints: Positioned within direct line of sight of nearby arrow slits, battlements, or machicolations so any intruders could be immediately targeted from above.