citadel vs spotter

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. 

spotter

noun
  • A member of a sniper team who in addition to this function is responsible for providing additional information about targets from a different point of view. 

  • A person who observes something. 

  • A team member responsible for communication with the driver to keep them up-to-date about on-track events, such as other nearby cars. 

  • A person who gives directions for a crane or vehicle (that is challenging to navigate) from a different point of view to the moved load. 

  • One who supervises a person performing an activity, in order to help them should they be unable to complete it. 

  • Dolphins of the genus Stenella, and specifically Stenella attenuata distinguished from the streaker (Stenella coeruleoalba) and the spinner (Stenella longirostris). 

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