city wall vs fencing

city wall

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

fencing

noun
  • Material used to make fences, fences used as barriers or an enclosure. 

  • Receiving and buying of stolen goods. 

  • The art or sport of duelling with swords, especially with the 17th- to 18th-century European dueling swords and the practice weapons descended from them (sport fencing). 

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