1. [n]  interchanging the positions of the king and a rook
2. [n]  a large building formerly occupied by a ruler and fortified against attack
3. [n]  (chess) the piece that can move any number of unoccupied squares in a direction parallel to the sides of the chessboard
4. [n]  a large and stately mansion
5. [v]  move the king two squares toward a rook and in the same move the rook to the square next past the king; in chess

1. \Cas"tle\, n. [AS. castel, fr. L. castellum, dim. of
   castrum a fortified place, castle.]
   1. A fortified residence, especially that of a prince or
      nobleman; a fortress.
   
            The house of every one is to him castle and
            fortress, as well for his defense againts injury and
            violence, as for his repose.          --Coke.
   
            Our castle's strength Will laugh a siege to scorn.
                                                  --Shak.
   
   Note: Originally the medi[ae]val castle was a single strong
         tower or keep, with a palisaded inclosure around it and
         inferior buidings, such as stables and the like, and
         surrounded by a moat; then such a keep or donjon, with
         courtyards or baileys and accessory buildings of
         greater elaboration a great hall and a chapel, all
         surrounded by defensive walls and a moat, with a
         drawbridge, etc. Afterwards the name was retained by
         large dwellings that had formerly been fortresses, or
         by those which replaced ancient fortresses.
   
   

2. \Cas"tle\, v. i. [imp. & p. p. {Castled}; p. pr. & vb. n.
   {Castling}.] (Chess)
   To move the castle to the square next to king, and then the
   king around the castle to the square next beyond it, for the
   purpose of covering the king.
   
   

a military fortress (1 Chr. 11:7), also probably a kind of tower used by the priests for making known anything discovered at a distance (1 Chr. 6:54). Castles are also mentioned (Gen. 25:16) as a kind of watch-tower, from which shepherds kept watch over their flocks by night. The "castle" into which the chief captain commanded Paul to be brought was the quarters of the Roman soldiers in the fortress of Antonia (so called by Herod after his patron Mark Antony), which was close to the north-west corner of the temple (Acts 21:34), which it commanded.