bulwark vs citadel

bulwark

noun
  • A defense or safeguard. 

  • A defensive wall or rampart. 

  • A breakwater. 

  • Any means of defence or security. 

  • The planking or plating along the sides of a nautical vessel above her gunwale that reduces the likelihood of seas washing over the gunwales and people being washed overboard. 

verb
  • To fortify something with a wall or rampart. 

  • To provide protection of defense for something. 

citadel

noun
  • A stronghold or fortified place. 

  • An armoured portion of a warship, housing important equipment. 

  • A Salvation Army meeting place. 

  • A strong fortress that sits high above a city. 

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