Since the 14th century, the Piccolomini Library, commissioned by Cardinal Francesco Piccolomini Todeschini, has housed the book heritage of Pope Pius II, including some great illuminated antiphonaries from the 15th-16th centuries. It is decorated with frescoes by Pinturicchio depicting the life of Aeneas Silvius Piccolomini. Over the years, the entire cathedral has been used as a museum and can be visited by purchasing a ticket at the entrance. Inside, one can admire magnificent architectural elements, such as the marble inlays on the floor, the pulpit by Nicola Pisano, the statue of St. John the Baptist by Donatello, the Piccolomini monument with some sculptures by Michelangelo, the Chigi chapel by Bernini, and other works.