'Strongholds' in the Bible

And David abode in the wilderness in the strongholds, and remained in the hill-country in the wilderness of Ziph. And Saul sought him every day, but God delivered him not into his hand.

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

And David went up from thence, and dwelt in the strongholds of En-gedi.