Postern

The postern of Newport Arch, built by the ancient Romans in Lincolnshire, England, located to the right of the larger main arch, and used for pedestrian traffic.

Postern in the rampart of Provins, Seine-et-Marne, France.

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.

The term is occasionally used in other contexts referring to a secondary door placed after a main entrance.