The Romanesque cathedral, the main moument of the city, is entitled to the patron St. Lawrence and was began at the end of the 13th century, by architect Sozzo Rustichini of Siena. Erected over the already existing church of Santa Maria Assunta, it was ended only in the course of the 15th century (mainly due to the unending struggles against Siena).
The façade of alternate layers of white and black marble appears of Romanesque style, but is almost entirely result of the 16th century and 1816-1855 restorations: of the originary buildings, it retains decorative parts including Evangelists' symbols. The plant is a Latin cross, with transept and apse. The interior has a nave with two aisles, parted by cruciform pilasters. The main artworks are a wondrously carved baptismal font from 1470-1474 and the Madonna delle Grazie by Matteo di Giovanni (1470).
The campanile (belfry tower) was finished in 1402, and restored in 1911.