The Collaborative International Dictionary of English v.0.48:

Terreplein \Terre"plein`\, n. [F., fr. L. terra earth + planus
   even, level, plain.]
   1. (Fort.) The top, platform, or horizontal surface, of a
      rampart, on which the cannon are placed. See Illust. of
      Casemate.
      [1913 Webster]

   2. (Civ. Engin.) An embankment of earth with a broad level
      top, which is sometimes excavated to form a continuation
      of an elevated canal across a valley.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

terreplein
    n 1: level space where heavy guns can be mounted behind the
         parapet at the top of a rampart