The entire inside structure is dominated by a colour dichotomy of black and white, in reference to the colours of Siena’s coat of arms.
The Cathedral’s inside is a real treasure trove of artistic masterpieces, from Nicola Pisano’s pulpit (1265-68), one of the most important sculpted pieces of the Italian 13th century, to Piccolomini’s altar, where one can admire four sculptures by the young Michelangelo: Saint Peter and Saint Paul, Saint Pious and Saint Augustine.
Just after the altar one finds the Piccolomini Library, commissioned in 1492 to store Pope Pious II’s extensive book collection. Its inside was decorated with frescoes from Pinturicchio, while in the chapel on the left one can admire Donatello’s famous San Giovanni Battista (St. John the Baptist), dating back to 1455.
The eight bronze statues adorning the pillars of the choir are works by Domenico Beccafumi. Above it one can see a copy of the famous stained-glass window of Duccio di Buoninsegna, completed in 1288, the oldest known stained glass of Italian manufacture. Finally, we have what may be the Cathedral’s most incredible piece: the opus sectile marble floor, a very unique piece of Italian art for its inventiveness, sheer scale and the importance of its collaborators.