'Fortresses' in the Bible

And David abideth in the wilderness, in fortresses, and abideth in the hill-country, in the wilderness of Ziph; and Saul seeketh him all the days, and God hath not given him into his hand.

And the Ziphites go up unto Saul to Gibeah, saying, 'Is not David hiding himself with us in fortresses, in the forest, in the height of Hachilah, which is on the south of the desolate place?

And David goeth up thence, and abideth in fortresses at En-gedi.