The Church and Sanctuary of the Most Holy Crucifix was built in the early 18th century to a design by architect Ferri in the space created by a steep slope below the Rocca di Federico. It is connected to the Palazzo Comunale by a scenic flight of steps. While the exterior decoration is sober and harmonious, the Greek-cross interior features walls completely frescoed with Scenes from the Life of Christ.
The vault of the drum dome is frescoed with an Ascension by Antonio Domenico Bamberini, while the pillars depict four episodes from the Old Testament.
On the high altar is an urn enclosed by a panel depicting the Risen Christ (1525) painted by Francesco Lanfranchi. The urn holds the 11th-century wooden Crucifix of Castelvecchio, an object of devotion by the faithful as it is believed to be miraculous. According to legend, it was abandoned in San Miniato by two wayfarers but saved the city from the plague of 1628. It was on this occasion that a vow was sanctioned, from which, at the behest of the bishop, a new Sanctuary was built to guard and venerate the sacred image of the miraculous Crucifix.