The Duomo's baptistery was built between 1355 and 1382 beneath the cathedral's choir and supports a Gothic facade left unfinished by Domenico di Agostino. The upper walls and vaulted ceilings inside were frescoed by Vecchietta and his school in the late 1440s (look for the alligator) but "touched up" in the 19th century. What you're principally here to see, though, is the baptismal font (1417-30). The frames are basically Gothic, but the gilded brass panels were cast by the foremost Sienese and Florentine sculptors of the early Renaissance. Starting on the side facing the altar, Siena's master Jacopo della Quercia did the Annunciation to Zacharias. Giovanni di Turino did the next two, the Birth of the Baptist and the Preaching of the Baptist. The Baptism of Christ is by the author of the Baptistery doors in Florence, Lorenzo Ghiberti, who collaborated with Giuliano di Ser Andrea on the Arrest of St. John. The final panel is perhaps the greatest, Donatello's masterful early study of precise perspective and profound depth in the Feast of Herod.