Hadrian's Mausoleum

The Mausoleum of Hadrian, now known as Castel Sant'Angelo is a towering cylindrical building in Parco Adriano, Rome. 
It was commissioned by the Roman Emperor Hadrian as a mausoleum for himself and his family. The building was later used by the popes as a fortress and castle. It is now a museum. The structure was once the tallest building in Rome.