This majestic building with an octagonal exterior and circular interior was begun in 1559 on a plan by Galeazzo Alessi and Giulio Danti, continued by Bino Sozi and concluded by Bernardino Sermigni in 1663. Sixteen columns support the entablature on which the 40-metre high dome rests. Behind the high altar is a fifteenth-century fresco of the Madonna with Child and two Saints, showing the influence of Gubbio; above the organ gallery is the Transfiguration by Nicholas Circignani, signed and dated (1578).