What is the meaning of Postern?

A back gate, back door, 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.

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

Situated at the rear; posterior.

definite singular of poster

Source: wiktionary.org