The cathedral, dedicated to San Lorenzo Martire in Santa Maria Assunta, was built from the last years of the thirteenth century, as evidenced by the inscription, nineteenth-century copy of a lost original, placed outside at the south-cornerwest, which recalls the Sienese architect Sozzo Rustichini, to be finished between 1330 and 1340.
The Duomo as it appears today is the result of a series of changes, even profound, that have occurred over time: first a restructuring of the sixteenth century, then a very extensive restoration that lasted throughout the nineteenth century.
The nineteenth-century intervention has tried to bring the entire building to an ancient Gothic purity: some baroque altars have been eliminated; a false cladding in bands, in imitation of the one in stone outside, was made with colored chalk on the pillars and arches. The facade, facing west, can be traced back to the general lines of the fourteenth century, but little dates back to that period. In particular, the sculptures representing the four evangelists should be attributed to the fourteenth-century phase.
The side facing Piazza Dante is largely original, except for the crowning of the portal, which remained unfinished, was completed in 1897.
Leaning on the north side of the Duomo is the bell tower, built in 1402, but raised one floor and radically modified in 1911.