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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

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: Webster's Revised Unabridged Dictionary (1913)