Constructed as a great circular tower surrounded by castle walls, Castel Sant’Angelo is the only castle located within the city of Rome. Built by Roman Emperor Hadrian XIV as his own mausoleum, the structure later came to be used as a castle. Contrary to its name, which means “Holy Angel,” this castle also had a dark side, serving as a prison for political and ideological criminals. Giordano Bruno, a thinker who espoused the heliocentric theory, was imprisoned here for seven years and then burned at the stake. A secret passage connected the castle to the Vatican, and the castle has been used as a place of refuge for the Pope. Today it serves as a military museum.