The co-cathedral contains many treasures, among them two splendid altarpieces, the fruit of the early activity of the Sienese painter Francesco Vanni, The Immaculate Conception and Saint John the Baptist in the Desert. The first altar in the right aisle holds The Immaculate Conception, dated 1588. The canvas, originally placed over the altar of the Immaculate Conception in the ancient Romanesque . . .