The Mausoleum of Hadrian, also known as Castel Sant'Angelo (Italian pronunciation: [kaˈstɛl sanˈtandʒelo]; English: Castle of the Holy Angel), is a towering rotunda (cylindrical building) in Parco Adriano, Rome, Italy. It was initially commissioned by the Roman Emperor Hadrian as a mausoleum for himself and his family. The popes later used the building as a fortress and castle, and it is now a museum. The structure was once the tallest building in Rome.