During the fall of the Roman Empire, some aqueducts were deliberately cut by enemies. In 537, the Ostrogoths laid siege to Rome, and cut the aqueduct supply to the city, including the aqueduct-driven grist-mills of the Janiculum. Belisarius, defender of the city, had mills stationed on the Tiber instead, and blocked the conduits to prevent their use by the Ostrogoths as ways through the city defences. In time, some of the city's damaged aqueducts were partly restored, but the city's population was much reduced and impoverished. Most of the aqueducts gradually decayed for want of maintenance, creating swamps and marshes at their broken junctions. By the late medieval period, only the Aqua Virgo still gave a reliable supply to supplement Rome's general dependence on wells and rainwater cisterns.