Sentence:  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 <WSD>posterne</WSD> / and there he mett with the two knyȝtes that chaced sir Blyaunt<!--Prior quote:Sir Launcelot ran oute at a <WSD>posterne</WSD>, and there he mette wyth tho two knyghtes that chaced Syr Blyaunte.--> 
Meaning of the word marked with <WSD> tags:  A back gate, back door, side entrance, or other gateway distinct from the main entrance.