Source : Webster's Revised Unabridged Dictionary (1913)

Castle \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.

Castle \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.

Source : WordNet®

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