city wall vs siege

city wall

noun
  • A wall around a city, built to define the city's boundaries, and sometimes to protect it from aggressors. 

siege

noun
  • A prolonged military assault or a blockade of a city or fortress with the intent of conquering by force or attrition. 

  • A period of struggle or difficulty, especially from illness. 

  • A prolonged assault or attack. 

  • The seat of a heron while looking out for prey. 

  • A flock of heron. 

verb
  • To assault or blockade a city or fortress with the intent of conquering by force or attrition; to besiege. 

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