Castel Sant'Elmo is a medieval fortress located on Vomero Hill adjacent to the Certosa di San Martino, overlooking Naples, Italy.

In 1329, using designs by the Sienese architect Tino da Camaino, king Robert of Naples enlarged the fortress described in documents as palatium in summitatae montanae Sancti Erasmi.

The Aragonese rulers of Naples, and notably Don Pedro de Toledo, the first governor and cousin of the Viceroy, included it in a comprehensive scheme designed to fortify the land perimeter of the city, based on four separate strongholds.

In fact, with its double tenaille, numerous embrasures in the bastions and high walls surrounded by a moat, the castle was admirably suited to the topography of the site and the strategic and defensive functions.

Around the parade grounds were situated the officers' quarters, chaplain's house, a church (1547) designed by the Spanish architect Pietro Prato, and the surviving buildings from the Angevin Belforte.

In 1982 the site was handed over to the Soprintendenza per i Beni Artistici e Storici of Naples, and the Bruno Molajoli Art History Library was installed in an upper story of the old prison block.