A city founded by the Gauls about the year 600 BC as Mediolanum, which became the capital of a Celtic tribe known as the Insubres. At the time of the Roman conquest, 222 BC, Mediolanum, was already one of the most powerful cities of the region on the Roman side of the Alps known as Cisalpine Gaul. Under the emperor Augustus, it became a part of the 11th region of Italy, acquiring increasing prestige and economic power until it became the second city of the Western Roman Empire behind Rome itself. In the 3rd century AD, following the division of the empire by Diocletian, it was the residence and main administrative center for one of the two emperors. Constantine the Great declared it the seat of the Vicar of Italy. In the year 452, Attila the Hun devastated the city, and in 539 the Goths destroyed it. The city sprung back to life by the second half of the 10th century. It was the principal road center of northern Italy and is the site of the imperial palace, Augustan and later walls, theater, amphitheater, circus, Constantinian baths, and early churches.