pos·tern 
(pōstərn, pŏstərn)
(pōstərn, pŏstərn)n.
A small rear gate, especially one in a fort or castle.
adj.
Situated in the back or at the side.
[Middle English posterne, from Old French, alteration of posterle, from Late Latin posterula, diminutive of Latin posterus, behind; see POSTERIOR.]