fort vs ravelin

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. 

ravelin

noun
  • An outwork. A fortification outside a castle used to split an attacking force; composed of two faces, forming a salient angle whose gorge resembles a half-moon 

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