barbican

noun4

1. •
   A tower at the entrance to a castle or fortified town.
2. •
   A fortress at the end of a bridge.
3. •
   An opening in the wall of a fortress through which the guns are levelled; a narrow loophole through which arrows and other missiles may be shot.

   • Two shafts of soft daylight fell across the flagged floor from the high barbacans. — 1922
4. •
   A temporary wooden tower built for defensive purposes.