postern

thumb|Postern doubling as a sallyport in the flank of a [[bastion at Dömitz Fortress in Germany]]

A postern is a secondary door or gate in a fortification such as a city wall or castle curtain wall. Posterns were often placed in concealed locations, allowing inconspicuous entrance and exit. 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.