Castel Sant'Elmo takes its name from the small church that stood here in the tenth century, St Erasmus - corrupted over the centuries to 'St Elmo'. A castle has stood here since 1329, when King Robert of Anjou modified a pre-existing Norman watchtower using the artists, architects and builders who were working on the adjacent monastery of San Martino. The castle acquired its massive six-pointed star shape in the mid 16th century, during an extensive reorganisation of city defences. In 1587 the ammunition dump was struck by lightning; the castle was badly damaged and 150 people died.
Over the years, the castle's dungeon has held many illustrious inmates, including heroes of the 1799 Parthenopean Republic, and it was a military prison until the mid 1970s. That might help to explain why the castle has never been a popular destination for Neapolitans. Its unmistakeable bulk has, however, long provided a focal point for pictorial cityscapes, including the extraordinary Tavola Strozzi in next door's Certosa-Museo di San Martino.
The ground floor of the castle is closed, although its gloomy interior can occasionally be glimpsed through the glass panels on the first floor - which is only opened during major exhibitions.
Piazza d'Armi, as the top floor is called, is in fact the castle's roof, and is best reached by lift; the 360° view over Naples is arguably the most breathtaking in the city. The walkway around the battlements is splendid, and a gently sloping path inside the castle leads you back down across the drawbridge, passing under the coat of arms of Carlo V.