burg vs castling

burg

noun
  • A fortified town in medieval Europe. 

  • burger 

  • A city or town. 

castling

noun
  • A move in which the king moves two squares towards a rook, and the rook moves to the other side of the king; the action of the verb to castle. 

  • One that is cast. 

  • A miniature cast or mould. 

  • The act of constructing a defense structure in Japanese chess in which the king (玉) is positioned in a certain way so that it is protected by pawns (歩) and silver general(s) (銀) and/or gold general(s) (金) often with an additional knight (桂) and lance (香車). 

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