A postern in a wall of the inner citadel. Byzantine, 7th century AD.

Postern = a secondary (small) door or gate, particularly 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.
On the other hand, a postern is too narrow to permit a successful attack from the outside.

Correspondent: J.M.Criel, Antwerpen