The Piccolomini Library, adjoining the duomo, was founded by Cardinal Francesco Piccolomini (afterwards Pius III.) in honour of his uncle, Pius II.
The Piazza del Duomo is at the north of the Corso.
The Opera del Duomo contains Duccio's famous Madonna, painted for the cathedral in 1308-1311, and other works of art.
The light and elegant tower (Torre del Mangla) soaring from one side of the palace was begun in 1338 and finished after 1348, and the chapel standing at its foot, raised at the expense of the Opera del Duomo as a public thank-offering after the plague of 1348, begun in 1352 and completed in 1376.