The Baptistery of San Giovanni Battista, flanked by an imposing stairway leading up to the level of the Piazza del Duomo, was built beneath the Cathedral, perhaps under the supervision of Camaino di Crescentino, capomastro (master builder) starting in 1299. It is decorated with a marble façade, left unfinished, which rises above the viewer, due to the limited space of the square in front of it. This imposing front covers not only the Baptistery, but also the choir section of the “old” cathedral which was expanded in this direction; the Cathedral’s enormous oculus, which once held the stained-glass window designed by Duccio (substituted in 2007 by a copy) is visible above it. The original window is now in the nearby cathedral museum called the Museo dell’Opera.
On 23 August 1339 the General Council of the Bell passed a resolution to enlarge Siena Cathedral: the existing church would become the transept of the new Cathedral, while the nave of this new church would cover the space that is now Piazza Jacopo della Quercia. In reality, already from 1 May 1317, work had begin on the expansion towards the east and construction begun on the Baptistery façade. In June 1325, when the vaults of the ceiling were closed, the main body of the Baptistery was essentially finished, as was the marble cladding on the façade and the sculptures in the area of the portals. After 1348, construction work on the so-called 'Duomo Nuovo' (New Cathedral) suffered a major slowdown and finally was suspended altogether, both because of the economic recession caused by the Black Plague and the problems of static stability that were arising in some of the sections that were already built.
During the expansion of the Cathedral, in the period from 1340 to 1348, when the sculptor Giovanni d’Agostino was appointed capomastro of the Opera di Santa Maria (the Cathedral’s administrative body), work continued on the Baptistery façade. The reliefs in the pediments of the Baptistery windows, with figures of the Blessing Christ, Saint Apollonia and a prophet, were sculpted by him in the 1340s. He also made the busts that decorate the pediments of the windows along the sides of the Baptistery (two on the left side and one on the right, of which copies are visible today). On the upper part of the façade, inserted into the pendentives of the archlets across the crown are eight very expressive and original heads of men and women, carved between the 1350s and 1365, the year when work on the Baptistery front was abandoned. It has been suggested that these were made by Giovanni d’Agostino’s brother Domenico, like Giovanni a sculptor and architect, who took over the job of master builder of Siena Cathedral after his brother’s death.
The pavement of the apron in front of the three portals is decorated with scenes of Birth, Baptism and Confirmation, made using the same technique of inlaid marble and graffito which was utilized for the famous floor of the cathedral. This iconography indicates the function of this sacred place, built to house the baptismal font.