Adam Clarke on Jeremiah 1:18

I have made thee this day a defenced city, and an iron pillar, and, brazen walls - Though thou shalt be exposed to persecutions and various indignities, they shall not prevail against thee. To their attacks thou shalt be as an impregnable city; as unshaken as an iron pillar; and as imperishable as a wall of brass. None, therefore, can have less cause to apprehend danger than thou hast. The issue proved the truth of this promise: he outlived all their insults; and saw Jerusalem destroyed, and his enemies, and the enemies of his Lord, carried into captivity. Instead of חמות chomoth, walls, many MSS. and editions read חמת chomath, a wall, which corresponds with the singular nouns preceding.