barbican vs crenellation

barbican

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

  • A fortress at the end of a bridge. 

  • A temporary wooden tower built for defensive purposes. 

crenellation

noun
  • A pattern along the top of a parapet (fortified wall), most often in the form of multiple, regular, rectangular spaces in the top of the wall, through which arrows or other weaponry may be shot, especially as used in medieval European architecture. 

  • The act of crenellating; adding a top row that looks like the top of a medieval castle. 

  • Any of a series of notches with fancied resemblance to such battlements, as for example around the bezel of a flashlight. 

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