embankment vs fortress

embankment

noun
  • a long mound of earth, stone, or similar material, usually built for purposes such as to hold back or store water, for protection from weather or enemies, or to support a road or railway. 

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