Mediolanum, the ancient city where Milan now stands, was originally an Insubrian city, but afterwards became an important Roman city in northern Italy. The city was settled by the Insubres around 600 BC, conquered by the Romans in 222 BC, and developed into a key centre of Western Christianity and informal capital of the Western Roman Empire. It declined under the ravages of the Gothic War, its capture by the Lombards in 569, and their decision to make Ticinum the capital of their Kingdom of Italy.

A section of Roman wall (11 m high) with a 24-sided tower

During the Principate the population was 40,000 in AD 200; when the city became capital of the Western Roman Empire under emperor Maximian (r. 286–305), the population rose to 100,000 people and thus Milan became one of the largest cities in Roman Italy.[1][2][3]