Castel Sant'Elmo is a medieval fortress located on Vomero Hill adjacent to the Certosa di San Martino, overlooking Naples, Italy. The name "Sant'Elmo" derives from a former 10th-century church dedicated to Sant'Erasmo, shortened to "Ermo" and, finally altered to "Elmo". Located near the upper terminus of the Petraio, one of the city's earliest pedestrian connections between upper and lower Naples, the fortress now serves as a museum, exhibition hall, and offices.