The concentrated human settlement that made possible — and required — writing, taxation, professional armies, law, specialization of labor, long-distance trade, public health infrastructure, and organized religion, and that has housed an increasing share of humanity since its first appearance in Mesopotamia, becoming the dominant form of human settlement in the 21st century for the first time in history.
Almost everything that distinguishes civilization from subsistence agriculture happened first in cities, and the city made it happen by doing one thing: concentrating people. Dense concentration creates surplus — labor surpluses, food surpluses, information surpluses. It creates division of labor, because enough people in one place means it pays to specialize. It creates disease, because enough people in one place means infection spreads. It creates politics, because enough people in one place means disputes require formal resolution. It creates inequality, because concentrated surplus attracts those with the power to extract it. The city did not invent these things — it made them necessary, which is the same as inventing them.