fastness vs fortress

fastness

noun
  • A fortified or secure place; a fortress, a stronghold. 

  • The ability of a dye to withstand fading. 

  • The quality or state of moving quickly; quickness, rapidity, swiftness. 

  • Something used to fasten or tie; a fastener or fastening. 

  • The quality or state of being strongly attached; firmness, secureness, tenacity. 

  • The quality or state of having an extravagant lifestyle or immoral habits. 

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