What's the noun for posterns? Here's the word you're looking for.
postern
A backgate, back door, side entrance, or other gateway distinct from the main entrance.
(archaic) By extension, a separate or hidden way in or out of a place, situation etc.
(historical, military) A subterraneanpassage communicating between the parade and the main ditch, or between the ditches and the interior of the outworks.
“Anteriority, or posteriority, is a characteristic of the Gothic, in that it gives expression to what Enlightenment culture pushes to the back.”
“This very loaded broaching, through the use of the word posthumanous, of the thought of an extreme posteriority finds itself sternly warned by Derrida's words, above.”
“Yet his writing is emptily abstract and opaque, e.g. As images of posteriority, ruins reveal the primordiality of the temporal law dial holds sway over their obsolescence.”