A postern is a secondary
door A door is a hinge A barrel hinge A hinge is a mechanical bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a ...

gate A gate or gateway is a point of entry to or from a space enclosed by wall A wall is a structure and a surface that defines an area; carries a load; provides security Security is freedom from, or resilience against, potential Potentia ...

in a
fortification A fortification is a military construction or building designed for the defense of territories in warfare, and is also used to establish rule in a region during peacetime. The term is derived from Latin ''fortis'' ("strong") and ''facere'' ( ...

such as a
city wall A defensive wall is a fortification A fortification is a military A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. It is typically officially authorize ...

city wall
castle A castle is a type of fortification, fortified structure built during the Middle Ages predominantly by the nobility or royalty and by Military order (monastic society), military orders. Scholars debate the scope of the word ''castle'', but u ...

curtain wall Curtain wall may refer to: * Curtain wall (architecture), the outer skin of a modern building * Curtain wall (fortification), the outer wall of a castle or defensive wall between two bastions