'Stronghold' in the Bible

But David took the stronghold of Zion, which is the city of David.

So David dwelt in the stronghold, and called it the city of David. And David built round about from the Millo and inward.

And the Philistines heard that they had anointed David king over Israel, and all the Philistines went up to seek David; and David heard of it, and went down to the stronghold.

And David was then in the stronghold; and the Philistines' garrison was then at Bethlehem.