The co-cathedral of San Secondiano – the Duomo di Chiusi – was built over the ruins of a medieval basilica from the 6th century, transformed and rebuilt in the 12th century. The building stands atop the foundations of a late-Roman building, as can be seen in the precious 5th-century mosaic that came to light during an excavation.
The interior of the basilica is separated into three large naves divided by 18 columns, recycled from older buildings, in marble and travertine with capitals in various styles. The atmosphere inside the church recalls the Early-Christian basilicas in Rome and truly makes one ponder the origins of Christianity. Indeed, Chiusi is one of the original centres of Christianity in Tuscany: here, you find the only catacombs in the Region.
The walls of the central nave, part of the counter-façade and the apse were painted to look like a Ravenna and Roman-style mosaic by the Sienese artist Arturo Viligiardi at the end of the 19th century. A complex iconographic program on the left and right sides of the central nave depict two series of martyrs buried in the Chiusi catacombs or tied to the historic tale of Saint Mustiola.
The bell tower is also noteworthy, which was built separately from the church in 1585 and later transformed into a defensive tower: below, around 12 meters deep, is a Roman pool from the 1st century BCE, made up of two barrel-vaulted rooms and which was also an ancient cistern.
The Cathedral Museum beside the church conserves a series of 21 illuminated manuscripts coming from the Abbey of Monte Oliveto Maggiore.