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:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.