citadel vs quarterdeck

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. 

quarterdeck

noun
  • The aft part of the upper deck of a ship; normally reserved for officers 

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