The meaning of Castle in the Bible
(From Easton's Bible Dictionary)

a military fortress (I Chronicles 11:7), also probably a kind of tower used by the priests for making known anything discovered at a distance (I 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.

See more on the meaning of Castle in the Bible:
Castle {International Standard Bible Encyclopedia}

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