In the event of a siege, a postern could act as a sally port, allowing defenders to make a sortie on the besiegers.
[1] Posterns were one of the essential means of ensuring safe communication between the enceinte and the outerworks of a defensive fortification.
[2] An 1850 West Point course summary on permanent fortifications discusses the placement and construction of posterns.
[1] 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.
[15] The term is occasionally used in other contexts referring to a secondary door placed after a main entrance.