citadel vs skyscraper

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. 

skyscraper

noun
  • A very tall building with a large number of floors. 

  • Anything very tall or high. 

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