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.

...
Wikipedia