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 Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.