'Fortress' in the Bible

"I've made you an assayer of my people, as well as a fortress. You know how to test their way."

To Moab: This is what the LORD of the Heavenly Armies, the God of Israel, says: "How terrible for Nebo, for it's laid waste. Kiriathaim is put to shame, it's captured. The fortress is put to shame, it's shattered.