postern

A small rear gate, especially one in a fort or castle.

Situated in the back or at the side.

A back door or gate; a private entrance; hence, any small door or gate. See cuts under castle and barbican.

In fortification, a covered passage closed by a gate, usually in the angle of the flank of a bastion, or in that of the curtain, or near the orillion, descending into the ditch.

Originally, a back door or gate; a private entrance; hence, any small door or gate.

A subterraneous passage communicating between the parade and the main ditch, or between the ditches and the interior of the outworks.

Back; being behind; private.

A back-gate, backdoor, side entrance, or other gateway distinct from the main entrance.

By extension, a separate or hidden way in or out of a place, situation etc.

Situated at the rear; posterior.