casern vs fortress

casern

noun
  • A lodging for soldiers in a garrison town (formerly usually near the rampart); a barracks. 

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 casern and fortress occurred in a corpus of books? (source: Google Ngram Viewer )