The Collaborative International Dictionary of English v.0.48:

Retirade \Ret`i*rade"\, n. [F.; cf. Sp. retirada retreat. See
   Retire.] (Fort.)
   A kind of retrenchment, as in the body of a bastion, which
   may be disputed inch by inch after the defenses are
   dismantled. It usually consists of two faces which make a
   reentering angle.
   [1913 Webster]