The Collaborative International Dictionary of English v.0.48:

Barbette \Bar*bette"\, n. [F. Cf. Barbet.] (Fort.)
   A mound of earth or a platform in a fortification, on which
   guns are mounted to fire over the parapet.
   [1913 Webster]

   En barbette, In barbette, said of guns when they are
      elevated so as to fire over the top of a parapet, and not
      through embrasures.

   Barbette gun, or Barbette battery, a single gun, or a
      number of guns, mounted in barbette, or partially
      protected by a parapet or turret.

   Barbette carriage, a gun carriage which elevates guns
      sufficiently to be in barbette. [See Illust. of
      Casemate.]
      [1913 Webster] Barbican

WordNet (r) 3.0 (2006):

barbette
    n 1: (formerly) a mound of earth inside a fort from which heavy
         gun can be fired over the parapet