Postern

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.

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