The Collaborative International Dictionary of English v.0.48:

Defilade \De`fi*lade"\, v. t. [imp. & p. p. Defiladed; p. pr.
   & vb. n. Defilading.] [Cf. F. d['e]filer to defile, and
   d['e]filade act of defiling. See 1st Defile.] (Mil.)
   To raise, as a rampart, so as to shelter interior works
   commanded from some higher point.
   [1913 Webster]

WordNet (r) 3.0 (2006):

defilade
    n 1: the arrangement of defensive fortifications to protect
         against enemy fire