Postern (Gormaz Castle)
A postern is a secondary door or gate in a fortification such as a city wall or castle.
The location of a postern was usually concealed - perhaps by bushes - and the idea was that it would allow the castle occupants to come and go without being seen. In the event of a siege they could nip out and surprise the enemy. Equally, once the besieging army knew of the whereabouts of the "side-door" they might try to use it as an entry point...