 Ever since, the Borromeos were the leading land owners (and at times Milanese governors) around the Lago Maggiore.Vitaliano Borromeo († 1449) had built a castle at Peschiera Borromeo near Milan in 1437.In 1450 Francesco I Sforza was backed by the family in his struggle to become heir and successor of the Visconti dukes and used the castle as a base for his siege of Milan.