But whatever confidence might be placed in ideal ramparts, the experience of the past, and the dread of the future, induced the Romans to construct fortifications of a grosser and more substantial kind. The seven hills of Rome had been surrounded by the successors of Romulus with an ancient wall of more than thirteen miles. The vast enclosure may seem disproportioned to the strength and numbers of the infant-state. But it was necessary to secure an ample extent of pasture and arable land against the frequent and sudden incursions of the tribes of Latium, the perpetual enemies of the republic. With the progress of Roman greatness, the city and its inhabitants gradually increased, filled up the vacant space, pierced through the useless walls, covered the field of Mars, and, on every side, followed the public highways in long and beautiful suburbs. The extent of the new walls, erected by Aurelian, and finished in the reign of Probus, was magnified by popular estimation to near fifty, but is reduced by accurate measurement to about twenty-one miles. It was a great but a melancholy labor, since the defence of the capital betrayed the decline of monarchy. The Romans of a more prosperous age, who trusted to the arms of the legions the safety of the frontier camps, were very far from entertaining a suspicion that it would ever become necessary to fortify the seat of empire against the inroads of the barbarians. The victory of Claudius over the Goths, and the success of Aurelian against the Alemanni, had already restored to the arms of Rome their ancient superiority over the barbarous nations of the North. To chastise domestic tyrants, and to reunite the dismembered parts of the empire, was a task reserved for the second of those warlike emperors. Though he was acknowledged by the senate and people, the frontiers of Italy, Africa, Illyricum, and Thrace, confined the limits of his reign. Gaul, Spain, and Britain, Egypt, Syria, and Asia Minor, were still possessed by two rebels, who alone, out of so numerous a list, had hitherto escaped the dangers of their situation; and to complete the ignominy of Rome, these rival thrones had been usurped by women.