Albert Barnes 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)

Metaphorically the walls and fortifications of the city represent the prophet’s power of patiently enduring the attacks of his enemies; while the iron pillar, supporting the whole weight of the roof (Judges 16:29; 1 Kings 7:21), signifies that no trials or sufferings would crush his steadfast will.