Milan is a very crowded city, with very little space. So when it came time to build a new church, St. Mary Near (or as I translate presso, up against) St. Satirus, the architects needed to be creative. St. Satirus was the brother of St. Ambrose, the archbishop of Milan.
In this photo by photographer Samoano from Wikimedia Commons, we see how the great Renaissance architect Donato Bramante created a trompe l’oeil apse since the street behind the church precluded building a real architectural one.
The sacristy is tucked away, and shows the influence of Bramante’s love of classical architecture. He would go on to design the Belvedere Palace in Rome, creating a style still copied today. Renaissance Revival architecture is partly inspired by Bramante’s work.