The Collaborative International Dictionary of English v.0.48:

Barbacan \Bar"ba*can\, n.
   See Barbican.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Barbican \Bar"bi*can\, Barbacan \Bar"ba*can\, n. [OE. barbican,
   barbecan, F. barbacane, LL. barbacana, barbicana, of
   uncertain origin: cf. Ar. barbakh aqueduct, sewer. F.
   barbacane also means, an opening to let out water, loophole.]
   1. (Fort.) A tower or advanced work defending the entrance to
      a castle or city, as at a gate or bridge. It was often
      large and strong, having a ditch and drawbridge of its
      own.
      [1913 Webster]

   2. An opening in the wall of a fortress, through which
      missiles were discharged upon an enemy.
      [1913 Webster] Barbicanage

WordNet (r) 3.0 (2006):

barbacan
    n 1: a tower that is part of a defensive structure (such as a
         castle) [syn: barbican, barbacan]