The Collaborative International Dictionary of English v.0.48:

Counterscarp \Coun"ter*scarp`\ (koun`t[~e]r*sk[aum]rp`) n.
   [Counter- + scarp: cf. F. contrescarpe.] (Fort.)
   The exterior slope or wall of the ditch; -- sometimes, the
   whole covered way, beyond the ditch, with its parapet and
   glacis; as, the enemy have lodged themselves on the
   counterscarp.
   [1913 Webster]