Built in 1368 by Gian Galeazzo Visconti to defend the city from attack, the castle was enlarged by military architect Magatti under the instruction of Visconti's successor Gian Galeazzo. Destroyed in 1447, it was rebuilt between 1450 and 1500 and enlarged under Francesco Sforza's rule. In 1500, once Sforza's rule was over, the building regained its military function and, after successive transformations, eventually fell into ruin. Rebuilt in the late 1900s, from 1893 onwards the castle gradually became home to various cultural institutions and art collections and remains so today. Guided tours are available.