fort vs redoubt

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. 

redoubt

noun
  • A small, temporary, military fortification. 

  • A reinforced refuge; a fort. 

  • The return of doubt. 

  • A place of safety or refuge. 

verb
  • To fortify, to make into a stronghold. 

  • To doubt again. 

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