Basilica of Saint Ambrogio is a masterpiece of the Lombard Romanesque architecture. It was founded in the last years of 4th century by Saint Ambrogio, one of the Doctors of the Church, renown for his extraordinary eloquence and for the authoritativeness which he demonstrated both in religious and political field. Then it was rebuilt between 11th and 12th century in romanesque style and its large atrium shows sculpture decorations rich of interesting details. The front, short and compact but lightened by large arcades, has on its sides two bell towers: the former and shorter from the 11th century, while the latter and higher was built in the 12th century. The portal of the church, renovated in the 18th century, still shows the original bronze leafs from the 9th century.

The interior keeps some important remains: in the nave there is a remarkable ambo in both romanesque and byzantin style; the main altar is covered with a rich gold frontal, masterpiece of the goldsmith art from Carolingian age. At the bottom of the right isle is visible the Chapel of St.Vittore, which contains some mosaics belongings to the age of the first basilica built in the 5th century. At the end of the left isle there is another entrance, built during Renaissance with the portal by Bramante.

Ambrogio, who was born in the German town of Treviri, belonged to an ancient noble family of Roman origin, always been at the emperor's service, and therefore he was an important imperial official. His intermediation was essential to the reconciliation of christians in Milan, divided by the Arianism heresy, which denied the divine origin of Christ and supported by the previous Bishop Assenzio. Milan was the imperial city chosen by Constantine for the issue in 313 of the famous edict which gave freedom of religion for Christians and it was therefore an highly symbolic seat. Ambrogio was elected bishop of Milan by acclamation, before being baptized yet.

St.Ambrogio renovated the liturgy of the mass and also the calendar of the church, with the so-called Ambrosian rite, which is still nowadays the rite followed by Milanese church. Thanks to his prestige, Agostino was converted to Christianity and the emperor Teodosio was obliged to do a public penance after the excommunication Ambrogio sent, since the emperor interfered politically with the Church's internal life without before consulting him. His tomb is contained in the crypt behind the chorus, togheter the ones by the saints Gervasio and Protasio.