Castel Sant'Angelo, also known as Mausoleum of Hadrian, is a towering rotunda 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 dedicated to Saint Michael the Archangel, and it is now a museum. The structure was once the tallest building in Rome.