citadel vs supply depot

citadel

noun
  • An armoured portion of a warship, housing important equipment. 

  • A stronghold or fortified place. 

  • A Salvation Army meeting place. 

  • A strong fortress that sits high above a city. 

supply depot

noun
  • A facility utilized by armies to store battlefield supplies temporarily on or near the front lines until they can be distributed to army units. 

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