postern

1. (architecture) A back gate, back door, side entrance, or other gateway distinct from the main entrance, especially in a city wall or fortification.Examples: "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."

2. (archaic, by extension) A separate or hidden way in or out of a place, situation etc.archaicbroadly

3. (historical, military) A subterranean passage communicating between the parade and the main ditch, or between the ditches and the interior of the outworks.Examples: "The postern of the enceinte leads through the middle of the curtain, descending from the plane of sight to the ditch"historical