donjon vs fort

donjon

noun
  • The fortified tower of a motte or early castle; a keep. 

fort

noun
  • A fortified defensive structure stationed with troops. 

  • Any permanent army post. 

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

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

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

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