fort vs fortress

fort

verb
  • To create a fort, fortifications, a strong point, or a redoubt. 

noun
  • Any permanent army post. 

  • An outlying trading-station, as in British North America. 

  • A structure improvised from furniture, bedding, etc., for playing games. 

  • A fortified defensive structure stationed with troops. 

fortress

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

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. 

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