bombproof vs stronghold

bombproof

noun
  • An air-raid shelter, a structure designed to give protection against air raids and bombs. 

verb
  • To make resistant to bomb blasts. 

adj
  • Sufficiently strong to resist the effects of a blast from a bomb. 

stronghold

noun
  • A place built to withstand attack; a fortress. 

  • A place of domination by, or refuge or survival of, a particular group or idea. 

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