The Castel Sant’Angelo

The Mausoleum of Hadrian, usually known as the Castel Sant’Angelo, is a towering cylindrical building in Rome, Italy. It was initially used by the Roman Emperor Hadrian as a mausoleum for himself and his family. Much of its decoration has been lost since the building’s conversion into a military fortress in 401 A.D. The catholic church used Sant'Angelo as a prison. Giordano Bruno was imprisoned there for six years. Now it is a popular museum.

Which castle ______?