Nineveh was the capital of Israel's cruel enemy, Assyria — the city Jonah did not want to warn. Yet when he finally preached there, the entire city, from king to commoner, repented, and God spared them.
Nineveh is a monument to the reach of God's mercy, which extends even to enemies, and a rebuke to any who would limit his compassion.
Nineveh in Scripture
So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.