Wanted by Francesco Sforza to strengthen the side of the castle facing the city and designed by the architect, Bartolomeo Gadio in 1452, these round towers clad in rusticated serizzo constituted a rather original piece of building on the Lombard scene. They were highly appreciated and cited by visiting nobility and ambassadors as an element that characterised the ancient manner. The towers, which contain six vaulted rooms and at one point housed prisons, were lowered in the 1500s and then again by the Austrians in 1848. The current roofing is the work of Beltrami, who restored the towers to their original height following the drawings made by the French engineering corps in 1800. The Carmine tower, which for some years was relegated to the function of a drinking-water cistern, today houses the Art Library.