Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. An opening in the wall of a fortress, through which
      missiles were discharged upon an enemy.

Source : WordNet®

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