The Co-Cathedral of San Secondiano is the cathedral of Chiusi and stands as one of the most fascinating examples of historical and architectural stratification in Tuscany. The building is immediately distinguishable from the outside by its freestanding bell tower, built in 1585 by converting an ancient defensive tower – a feature that bears witness to the link between religious architecture and the medieval urban fabric.

Beneath the bell tower, at a depth of around twelve metres, lies a surprising remnant of the Roman era: a 1st-century BC pool, comprising two rooms with barrel vaults, which reveals the site’s continuous use since antiquity.

Archaeological excavations carried out in the 1970s brought to light, beneath the chancel, the remains of an earlier basilica, built between the late 4th and early 5th centuries. This early Christian building, characterised by three naves with square pillars, featured walls decorated with painted plaster and a mosaic floor. The present co-cathedral was rebuilt around the mid-6th century, reusing part of the structures and materials from the ancient complex.

The interior follows a basilica-style layout with three naves, marked by eighteen columns taken from older, probably Roman, buildings. Crafted from marble and travertine, with capitals in various styles, these columns bear witness to the building’s long history, spanning the early Christian era, Renaissance alterations and 19th-century restorations. A detail of great interest is the inscription on the third column from the left, which bears the name of Bishop Florentino, allowing the reconstruction to be dated between 558 and 560.

The central nave is lit by eight large windows on each side and decorated according to a complex iconographic scheme: along the walls, figures of saints and martyr saints follow one another, linked to the catacombs of Chiusi or the tradition of Saint Mustiola. In the apse, a large painting created in 1892, inspired by the mosaics of the Basilica of Santa Maria Maggiore, completes the decorative scheme, flanked by depictions of Saint Ursula and Saint Catherine of Alexandria.

The Co-Cathedral of San Secondiano is today a place of extraordinary historical and spiritual value, where different eras overlap in a continuous dialogue between the Roman world, early Christianity and the art of subsequent centuries.