Definition of "postern"

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(please add an English translation of this quotation)

1485, Sir Thomas Malory, “ij”, in Le Morte Darthur, book XII

He by a privy postern took his flight.

1590, Edmund Spenser, “Book I, Canto VI”, in The Faerie Queene. […], London: […] [John Wolfe] for William Ponsonbie, stanza 52

Out at the postern, by the abbey wall.

c. 1590–1591 (date written), William Shakespeare, “The Two Gentlemen of Verona”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act V, scene i]

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

1850, Dennis Hart Mahan, Summary of the Course of Permanent Fortification and of the Attack and Defence of Permanent Works