barbican vs breastwork

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. 

breastwork

noun
  • A fortification consisting of a breast-high bulwark; a parapet. 

  • A parapet. 

  • Breast augmentation. 

  • A railing on the quarter-deck and forecastle. 

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