'Strongholds' in the Bible

They put him in a cage with hooks, and brought him to the king of Babylon; they brought him into strongholds, that his voice should no more be heard on the mountains of Israel.

You shall tell them, Thus says the Lord Yahweh: As I live, surely those who are in the waste places shall fall by the sword; and him who is in the open field will I give to the animals to be devoured; and those who are in the strongholds and in the caves shall die of the pestilence.