A military fortress (1 Chronicles 11:7), also probably a kind of tower used by the priests for making known anything discovered at a distance (1 Chronicles 6:54). Castles are also mentioned (Genesis 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.

Noah Webster's New International Dictionary of the English Language

1. (n.) A fortified residence, especially that of a prince or nobleman; a fortress.

2. (n.) Any strong, imposing, and stately mansion.

3. (n.) A small tower, as on a ship, or an elephant's back.

4. (n.) A piece, made to represent a castle, used in the game of chess; a rook.

5. (v. i.) 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.