San Nazaro in Brolo church was built by St. Ambrose starting from 382 on the road that connected Milan (then Mediolanum) to Rome. It was originally dedicated to the Apostles, and thus known as Basilica Apostolorum.
As explained by an inscription in the church written by Ambrose himself, the church's plan was on the Greek Cross with apses on the arms, a feature present only in the Church of the Holy Apostles in Constantinople. In front of the basilica was a porticoed atrium. Under the basilica's altar were housed the relics of the Apostles, which are still present. In 397, when the body of St. Nazarus was discovered, a new apse was created. Serena, niece of emperor Theodosius I, donated the marbles for the sacellum housing the relics and also embellished the rest of the church.
The apse of the right arm has a portal with a false porch. The ceiling of the nave, originally consisting of wooden spans, was replaced by a groin vault during the Middle Ages. The walls are original. Also in this age the Romanesque-style octagonal tambour, featuring a circular loggia with small columns, was added over the arms' crossing.
Starting from 1512, Bramantino built the Trivulzio Mausoleum, which obstructs the Palaeo-Christian façade.
References:Visby Cathedral (also known as St. Mary’s Church) is the only survived medieval church in Visby. It was originally built for German merchants and inaugurated in 1225. Around the year 1350 the church was enlarged and converted into a basilica. The two-storey magazine was also added then above the nave as a warehouse for merchants.
Following the Reformation, the church was transformed into a parish church for the town of Visby. All other churches were abandoned. Shortly after the Reformation, in 1572, Gotland was made into its own Diocese, and the church designated its cathedral.
There is not much left of the original interior. The font is made of local red marble in the 13th century. The pulpit was made in Lübeck in 1684. There are 400 graves under the church floor.