citadel vs observatory

citadel

noun
  • A strong fortress that sits high above a city. 

  • A stronghold or fortified place. 

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

  • A Salvation Army meeting place. 

observatory

noun
  • A lookout (vantage point with a view of the surrounding area) 

  • A place where stars, planets and other celestial bodies are observed, usually through a telescope; also place for observing meteorological or other natural phenomena. 

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