Hotel Campo Imperatore in Gran Sasso was Benito Mussolini's prison until his freeing on September 1943 by Nazi commandos in the Gran Sasso raid. The plateau is also the site of the Campo Imperatore station of the Rome Observatory, from which the Campo Imperatore Near-Earth Object Survey and other astronomical studies are carried out. At the southern edge of Campo Imperatore and within the bounds of the national park are three medieval hill towns once ruled by the Medicis; Calascio, which sits before the ancient fortress ruin of Rocca di Calascio, Santo Stefano di Sessanio, and Castel Del Monte.