battlement vs fortress

battlement

noun
  • In fortification: an indented parapet, formed by a series of rising members called cops or merlons, separated by openings called crenelles or embrasures, the soldier sheltering himself behind the merlon while he fires through the embrasure or through a loophole in the battlement. 

  • The towering roof of heaven. 

  • Any high wall for defense. 

fortress

noun
  • A fortified place; a large and permanent fortification, sometimes including a town; for example a fort, a castle; a stronghold; a place of defense or security. 

  • A position that, if obtained by the weaker side, will prevent penetration by the opposing side, generally achieving a draw. 

verb
  • To furnish with a fortress or with fortresses; to guard, to fortify. 

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