barbican vs parapet

barbican

noun
  • A tower at the entrance to a castle or fortified town 

  • 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. 

  • A fortress at the end of a bridge. 

  • A temporary wooden tower built for defensive purposes. 

parapet

noun
  • A fortification consisting of a wall. 

  • Part of a perimeter that extends above the roof. 

  • A low protective wall. 

  • Hyponym: balustrade 

How often have the words barbican and parapet occurred in a corpus of books? (source: Google Ngram Viewer )