It is the biggest and most important Romanesque church in Tuscany. Its construction in the medieval town was begun on the second half of the XII century and was completed during the first years of the XIV century. It was then restored many times in the XVI, XIX and XX centuries. Its facade, in a Pisa style, dates back to the beginning of the XIII century. It is made up of five blind arches overlooked by three lines of loggias with small columns. On the main door there are sculptures of the XIII century, dedicated to the Virgin, and on the archivolt there is the Month’s Cycle dating back to the same period. The horizontal movement of the facade clashes against the vertical tenden­cy of the tower (finished in 1330), which is called ‘of the hundred holes”, owing to its ma­ny “bifore” win­dows underlin­ing its height.
In the interior there are three naves and a great apse (restored between 1862 and 1875), pilaster columns, and a wooden truss-ceiling. The inner side of the facade bears a marble bass-relief depicting the Worship of the Magi. Underneath the tower there is a baptismal font of the XVI century. The  Presbytery overlooks the five-nave crypt, restored in the XIX century, which  preserves the reliquary bust of S. Donato, Patron Saint of the town.
On the high altar there is the Polyptychon, painted by Pietro Lorenzetti for the Bishop Guido Tarlati in 1320: it represents the Madonna with Child, Annunciation and  Saints.
Along the left wall there is the Sacrament Chapel, decorated by Ademollo.