Classical Antiquity can generally be defined as an age of cities; the Greek polis and Roman municipium were locally organised, self-governing bodies of citizens governed by written constitutions. When Rome came to dominate the known world, local initiative and control were gradually subsumed by the ever-growing Imperial bureaucracy; by the Crisis of the Third Century the military, political and economic demands made by the Empire had crushed the civic spirit, and service in local government came to be an onerous duty, often imposed as punishment. Harassed urban dwellers fled to the walled estates of the wealthy to avoid taxes, military service, famine and disease. In the Western Roman Empire especially, many cities destroyed by invasion or civil war in the 3rd century could not be rebuilt. Plague and famine hit the urban class in greater proportion, and thus the people who knew how to keep civic services running. Perhaps the greatest blow came in the wake of the extreme weather events of 535–536 and subsequent Plague of Justinian, when the remaining trade networks ensured the Plague spread to the remaining commercial cities. The impact of this outbreak of plague has recently been disputed. The end of Classical Antiquity is the end of the Polis model, and the general decline of cities is a defining feature of Late Antiquity.