'Fortification' in the Bible

"I have made you an assayer among my people, a fortification, so that you may know and put to the test their ways.

when the army of the king of Babylon [was] fighting against Jerusalem and against all the cities of Judah that were left over--Lachish and Azekah, for these remained among the cities of Judah, the cities of fortification.

And it cometh to pass, in the ninth year of his reign, in the tenth month, in the tenth of the month, come hath Nebuchadrezzar king of Babylon -- he and all his force -- against Jerusalem, and they encamp against it, and build against it a fortification round about;