Charles Ellicott Commentary Jeremiah 1:18

"For, behold, I have made thee this day a fortified city, and an iron pillar, and brazen walls, against the whole land, against the kings of Judah, against the princes thereof, against the priests thereof, and against the people of the land." — Jeremiah 1:18 (ASV)

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 invading armies. But the outcome is different. They fall: he will hold out.

The iron pillar is that which, rising in the center 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 people may meet.

The “brasen walls” probably refer to the practice of fastening plates of copper over the brick or stonework of a fortification.