The cathedral's valuable pieces of art including The Feast of Herod by Donatello, and works by Bernini and the young Michelangelo make it an extraordinary museum of Italian sculpture.
The main attraction of the baptistry  is the hexagonal baptismal font, containing sculptures by Donatello, Jacopo della Quercia and others. The panel of is one of the great masterpieces of Renaissance sculpture. It was the first relief to be built in accordance with the rules of perspective.
The relief of , at Siena, was executed by Donatello in 1425-1427. It is one of two panels originally ordered from Jacopo della Quercia for the baptismal fonts of Siena Cathedral. Here the architectural setting acts as one of the principal motifs of the scene. Possibly this setting was designed by Michelozzo. At any event it stands out in the history of art as the first relief to be built up in accordance with the rules of perspective. (...) This strict perspective layout and the network of straight lines structuring it, heighten the dramatic effect of the scene. Starting from the Baptist's severed head presented on a salver to the horrified Herod, arises the crescendo of rhythmed gestures conveying the emotional response of the figures, expressed already by contorted or spirited movements, by the restless animation of the drapery. The upsweep of her dress shows us Salome still dancing. The memory of her slender, buoyant figure lingers on in Lippi and Botticelli.
Donatello (ca. 1386–1466) - Metropolitan Museum of Art
Donatello is responsible for yet another trend in art. The equestrian sculpture. Prior to Donatello's creation of the equestrian sculpture Gattamelata in 1443, equestrian statues were the sole prerogative of the ruling class. Despite the resounding scandal that accompanied the creation of the horse and rider sculpture Donatello's work was well received and even before its official unveiling he had offers to make other versions of the sculpture. The modern equestrian sculpture as we know it today was popularized and is entirely indebted to Donatello.
Donatello's David - SUNY Oneonta
Major works by the Renaissance sculptor Donatello (c. 1386-1466) are rarely seen in the U.S. Most of his oeuvre, monumental in size and fragile, remains in Italy—especially the statuary he executed for Florence’s magnificent cathedral of Santa Maria del Fiore, the celebrated Duomo. Today, copies of those works and those by his circle embellish the Duomo façade. The originals are preserved in Florence’s Museo dell’ Opera del...
Donatello - Biography and Gallery of Art