Jeremiah 1:18

I have made thee... a defenced city... — Images of strength are heaped one upon another. The prophet is represented as attacked by kings, princes, priests, and people, as the cities of Judah are by the invading armies. But the issue is different. They fall: he will hold out. The iron pillar is that which, rising in the centre of an Eastern house or temple (as, e.g., in Judges 16:25; 1 Kings 7:21), supports the flat roof, and enables it to be used as a terrace or platform on which men may meet. The “brasen walls” probably refer to the practice of fastening plates of copper over the brick or stonework of a fortification.