Tower vs prison camp

Tower

noun
  • Denoting the system of weights used by the Saxon and Norman English kings in their minting of coins. 

name
  • The Tower of London, especially seen as a place of imprisonment or punishment. 

  • A habitational surname.; Alternative form of Towers 

prison camp

noun
  • A camp in which either prisoners of war or political prisoners are interned. 

  • A low-security federal prison. 

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