Known locally as the Maschio Angioino, this forbidding castle was built by Charles I of Anjou in 1279 and then used as a royal residence by successive Angevin monarchs; the magnificent carved triumphal arch was added in the 15th century. Today the castle houses the city council offices and two museums. You can also visit the Sala dei Baroni, which has a beautiful ceiling, and the Cappella Palatina with its fresco fragments by Giotto.