On the square where the cathedral now stands, called Piano Sancte Marie (the plain of Saint Mary) in medieval documents, as early as the tenth century there was already a little church, located next to the bishop’s residence and oriented in the opposite direction from the current cathedral, with its façade looking towards where the apse is today.
During the twelfth century, this church was encompassed in a larger building, turned ninety degrees with respect to the earlier one (facing the square now called Piazza Jacopo della Quercia), until, in the early decades of the thirteenth century, another transformation was set in motion which resulted in the basilica you see now, with the façade definitively oriented towards the Santa Maria della Scala hospital.
We learn from some payment records that in 1263 the dome had already been finished, covered with plates of lead and topped by a copper gilt ball, commonly called the ‘apple.’ The lantern supporting it is the result of a seventeenth-century renovation.
By 1284 the nave and side aisles had been built and construction had begun on the façade under the direction of Giovanni Pisano, who worked on it until 1297, finishing the lower section. After he was sent away from Siena, the project was finished twenty years later by Camaino di Crescentino, who took Giovanni’s place as master builder of the cathedral. In the meantime, on the remains of a tower of the fortified castle that had dominated the square in the early Middle Ages, the bell tower had been built and sheathed, like the rest of the church, in alternating bands of white and black (or more precisely, dark green) marble, in keeping with the taste typical of Tuscan Romanesque.
Towards the end of the second decade of the fourteenth century, work was begun to enlarge the choir and transept, until, in 1339, when the city was at the height of its splendor, a very ambitious project was initiated, which called for the construction of a new, gigantic house of worship, for which the existing church would become the transept. Less than a decade later, in 1348, a terrible epidemic of Black Plague, together with the collapse of part of the new building because of structural problems, put an end to Siena’s dream. The memory of that great enterprise still remains in the incomplete structures that trace the outline of Piazza della Quercia: the imposing façade, called by the Sienese the “facciatone,” and the pillars topped by arches that would have separated the central nave from the side aisles. On the right side of what would have been the new cathedral is an elegant portal, intended to be the church’s side door, beautifully decorated with sculptures by Giovanni d’Agostino.
After the failure of this project, work resumed on the existing building, and in 1370 the cathedral could be said to be finished.