The Collaborative International Dictionary of English v.0.48:

Caponiere \Cap`o*niere"\, n. [F. caponni[`e]re, fr. Sp.
   caponera, orig., a cage for fattening capons, hence, a place
   of refuge; cf. It. capponiera. See Capon.] (Fort.)
   A work made across or in the ditch, to protect it from the
   enemy, or to serve as a covered passageway.
   [1913 Webster]