Locals know this 13th-century castle as the Maschio Angioino (Angevin Keep) and its Cappella Palatina is home to fragments of frescoes by Giotto; they're on the splays of the Gothic windows. You'll also find Roman ruins under the glass-floored Sala dell'Armeria (Armoury Hall). The castle's upper floors (closed on Sunday) house a collection of mostly 17th- to early-20th-century Neapolitan paintings. The top floor houses the more interesting works, including landscape paintings by Luigi Crisconio and a watercolour by architect Carlo Vanvitelli.
The history of the castle stretches back to Charles I of Anjou, who upon taking over Naples and the Swabians' Sicilian kingdom found himself in control not only of his new southern Italian acquisitions but also of possessions in Tuscany, northern Italy and Provence (France). It made sense to base the new dynasty in Naples, rather than Palermo in Sicily, and Charles launched an ambitious construction program to expand the port and city walls. His plans included converting a Franciscan convent into the castle that still stands in Piazza Municipio.
Christened the Castrum Novum (New Castle) to distinguish it from the older Castel dell'Ovo and Castel Capuano, it was completed in 1282, becoming a popular hang-out for the leading intellectuals and artists of the day – Giotto repaid his royal hosts by painting much of the interior. Of the original structure, however, only the Cappella Palatina remains; the rest is the result of Aragonese renovations two centuries later, as well as a meticulous restoration effort prior to WWII.
The two-storey Renaissance triumphal arch at the entrance – the Torre della Guardia – commemorates the victorious entry of Alfonso I of Aragon into Naples in 1443, while the stark stone Sala dei Baroni (Hall of the Barons) is named after the barons slaughtered here in 1486 for plotting against King Ferdinand I of Aragon. Its striking ribbed vault fuses ancient Roman and Spanish late-Gothic influences.