A postern is a secondary door or gate in a fortification such as a city wall or castle curtain wall. Posterns were often located in a concealed location which allowed the occupants to come and go inconspicuously. In the event of a siege, a postern could act as a sally port, allowing defenders to make a sortie on the besiegers. Placed in a less exposed, less visible location, they were usually relatively small, and therefore easily defensible.
Posterns were one of the essential means of ensuring safe communication between the enceinte and the outerworks of a defensive fortification. An 1850 West Point course summary on permanent fortifications discusses the placement and construction of posterns.
In literature, a postern features in the Le Chanson de Girart de Roussillon, where the hero makes use of one to escape when betrayed; as does Renaud de Montauban in the chanson de geste, The Four Sons of Aymon. A postern also provided a safe retreat for Ogier the Dane.
In Malory's Le Morte d'Arthur, "La Cote de Male Tayle" is rescued at the Castle Orgulous when a damsel slips through the postern to find his horse and ties it to the postern so that La Cote de Male Tayle can escape the 100 knights assailing him.
The term is occasionally used in other contexts referring to a secondary door placed after a main entrance.
The postern of Newport Arch, built by the ancient Romans in Lincoln, England, located to the right of the larger main arch, and used for pedestrian traffic