Fortress

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 18:1919A brother offended is harder to be won than a strong city: and their contentions are like the bars of a castle. (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. 5:6-76And the king and his men went to Jerusalem unto the Jebusites, the inhabitants of the land: which spake unto David, saying, Except thou take away the blind and the lame, thou shalt not come in hither: thinking, David cannot come in hither. 7Nevertheless David took the strong hold of Zion: the same is the city of David. (2 Samuel 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, 3734And some cried one thing, some another, among the multitude: and when he could not know the certainty for the tumult, he commanded him to be carried into the castle. (Acts 21:34)
37And as Paul was to be led into the castle, he said unto the chief captain, May I speak unto thee? Who said, Canst thou speak Greek? (Acts 21:37)). 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. 18:22The Lord is my rock, and my fortress, and my deliverer; my God, my strength, in whom I will trust; my buckler, and the horn of my salvation, and my high tower. (Psalm 18:2); Psa. 31:33For thou art my rock and my fortress; therefore for thy name's sake lead me, and guide me. (Psalm 31:3); Psa. 71:33Be thou my strong habitation, whereunto I may continually resort: thou hast given commandment to save me; for thou art my rock and my fortress. (Psalm 71:3); Psa. 91:22I will say of the Lord, He is my refuge and my fortress: my God; in him will I trust. (Psalm 91:2)).
 
A fortress on the Red Sea.