Webster's English Dictionary

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castle

1. cas.tle \'kas-*l\ n [ME castel, fr. OE, fr. ONF, fr. L castellum 
   fortress, castle, dim.] of castrum fortified place; akin to L castrare to 
   castrate 1a: a large fortified building  1b: a massive or imposing house  
   2: a retreat safe against intrusion or invasion  3: ROOK 
2. castle \'kas-(*-)lin\ vb or cas.tled;  or cas.tling 1: to establish in a 
   castle  2: to move (the king in chess) in castling  1: to move the king two 
   squares toward a rook and in the same move the roo k to the square next 
   past the king of the king  2: to move in castling