'Strongholds' in the Bible

And the men of Israel saw that they were in a strait (for the people were distressed); and the people hid themselves in caves, and in thickets, and in cliffs, and in strongholds, and in pits.

And David abode in the wilderness in strongholds, and abode in the mountain in the wilderness of Ziph. And Saul sought him every day, but God did not give him into his hand.

And the Ziphites came up to Saul to Gibeah, saying, Does not David hide himself with us in strongholds in the wood, on the hill of Hachilah, which is on the south of the waste?

And David went up from thence, and abode in the strongholds of Engedi.