Tower vs gulag

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 

gulag

verb
  • To compel (someone) into a forced labour camp or a similar place of confinement or exile. 

noun
  • A prison camp, especially one used to hold political prisoners. 

  • Also GULAG: the system of all Soviet labour camps and prisons in use, especially during the Stalinist period (1930s–1950s). 

  • A place where, or political system in which, people with dissident views are routinely oppressed. 

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