The rustic stone façade was built in Romanesque style using spolia from the Roman theater of Volterra in the portal.

[1] In 1580, bishop Guido Serguidi, with the approval of Ferdinando I de' Medici, Grand Duke of Tuscany, commissioned the decoration of the intricately coffered ceiling of the nave and crossings, designed by Francesco Capriani, carved by Jacopo Pavolini, and gilded by Fulvo della Tuccia.

The ceiling has sculpted reliefs of Saints Ugo, Giusto, Pope Linus, Clemente, Attinea, and Greciniana, round a central symbol of the Holy Spirit.

In the 19th century, the walls of the interior were painted in stripes that suggest typical Tuscan decorative church architecture.

The pulpit has three reliefs depicting the Last Supper, the Annunciation, and the Sacrifice of Isaac, and the lions at the base of the columns are undoubtedly 12th-century works of the Guglielmo school, perhaps carved by the Pisan sculptor Bonamico.

In the ceiling of the crossing are depictions of God the Father and Stories of the Life of the Virgin (1585) above the choir by Niccolò Circignani.

It was commissioned in 1522 to thank the saint for his intervention in stopping an epidemic of the plague; the candle holders depicting angels are by Andrea Ferrucci.

The admiral, in a scene including the Baptistery of Volterra, appears in conversation in the lunette of the Procession to Damascus above the altar.

The figure of the prelate dressed in his finery in a sleeping pose on the sarcophagus was executed by Giovanni Angelo Montorsoli, one of Michelangelo's assistants.

The Oratory of the Virgin Mary (Cappella dell'Addolorata) contains a fresco of the Nativity with a Journey of the Magi by Benozzo Gozzoli on the left and an Epiphany on the right.

The Chapel of the Holiest Name of Jesus has an architectural 16th-century frame preserving the monogram of Christ donated to Volterra by San Bernardino of Siena encased in a precious silver 18th-century shrine.

Above the altar is a canvas of the Ecstasy of St Carlo Borromeo before the Virgin by Jacopo Chimenti, and on the side walls Saint Mary Magdalene of Scolaro by Guido Reni and an Immaculate Conception by Francesco Brini.