Boyd’s Bible Dictionary:
A fortress on N. W. side of Temple at Jerusalem (Acts 21:31-40).
Concise Bible Dictionary:
A tower or fortress built by Herod the Great near the temple at Jerusalem in which he placed a guard to watch over the approaches to the sacred edifice. Josephus (Wars 5. 5, 8) says it was situated “at the corner of two cloisters of the court of the temple; of that on the west, and that on the north; it was erected upon a rock fifty cubits in height, and was on a great precipice.” Where this precipice was is not known, for it is a much disputed question upon what part of the temple area the temple was built. There is a tower, now called Antonia, on the N. W. angle, and there are indications of a similar one having stood on the S. E. angle.