Concise Bible Dictionary:
The terms “fortress,” “stronghold,” and “castle” mostly refer to a part of a city that was more strongly fortified than by the mere walls. Proverbs 18:19 speaks of the “bars of a castle.” There was such a place in Jerusalem when the city was taken by David, which was held by the Jebusites (2 Sam. 5:6-7). The Romans had a “castle” in Jerusalem, to which Paul was carried when he was seized by the Jews (Acts 21:34, 37). This may have been the same that was called ANTONIA, a fortress built by Herod the Great, adjoining the temple, as described by Josephus: Wars, v. 5. 8. The Psalmist often calls Jehovah his rock and fortress (Psa. 18:2; Psa. 31:3; Psa. 71:3; Psa. 91:2).
A fortress on the Red Sea.