postern

noun

1. A back gate, back door, side entrance, or other gateway distinct from the main entrance, especially in a city wall or fortification.

   “And as they cam hurlyng vnder the Castel where as sir launcelot lay in wyndowe / & sawe how two knyghtes layd vpon syr Blyaunt with their swerdes /[…]/ And thenne sir launcelot brake the chaynes fro his legges and of his armes /[…]/ & so sir launcelot ran out at a posterne / and there he mett with the two knyȝtes that chaced sir Blyaunt”

   “He by a privy postern took his flight.”

   “Out at the postern, by the abbey wall.”

   “The Duke of Argyle led the way in silence to the small postern by which they had been admitted into Richmond Park, so long the favourite residence of Queen Caroline. It was opened by the same half-seen janitor, and they found themselves beyond the precincts of the royal demesne.”

2. (archaic, broadly)A separate or hidden way in or out of a place, situation etc.
3. (historical)A subterranean passage communicating between the parade and the main ditch, or between the ditches and the interior of the outworks.

   “The postern of the enceinte leads through the middle of the curtain, descending from the plane of sight to the ditch”

adj

1. Situated at the rear; posterior.