In 222 BC the Romans conquered the Celtic settlement and in 286 BC Milan, then called Mediolanum, became the capital of the Western Roman empire under Diocletian. In 313 AD the emperor Constantine the first, through the Edict of Milan guaranteed to the Christians freedom of religion. In 374 AD Ambrosgio became the archbishop of Milan by acclamation. The people wanted him to lead the Milanese church and he became priest on the 7th of December. This day is still a local holyday for the city of Milan. During his mandate Ambrosius commissioned the construction of three churches: the Basilica Ad Martyres (todays it is is known as Basilica di Sant’Ambrogio), the Basilica degli Apostoli (San Nazaro) and the Basilica delle Vergini (San Simpliciano).
During the Roman domination Milan became a thriving city. The construction of these churches demonstrates how the Milanese were masters in art, culture and architecture. The city also featured an imperial palace (its ruins are still visible in Via Brisa), the walls and the circus (Circo) which was destroyed during the invasion of Frederick the first in 1162. The ruins of the circus have been found in the basement of some buildings in via Circo. In 402 AD the barbarians crossed the Alps and by 472 AD the city of Milan is conquered and partially destroyed. It was the end of the Western Roman Empire and the beginning of the Middle Ages.