This lovely church just outside the walls, completed in 1534, is the masterwork of Antonio da Sangallo the Elder and one of the treasures of High Renaissance architecture. Sangallo was best known for fortresses and other military defenses, but here he broke out of the mold to create a beautiful travertine church of classical proportions on the plan of a Greek cross, with the four arms of equal length radiating from a central dome. Since the church is in open countryside with no other buildings nearby, it’s easy to admire the classical unity of the place, and that is all you need to do—the interior is as refined as the exterior but like many a great beauty, a bit dull at close inspection.