Also known as the Mausoleum of Hadrian, ( English: Castle of the Holy Angel), is a towering cylindrical building in Rome.
It was initially 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, and is now a museum. The structure was once the tallest building in Rome.
Hadrian also built the Pons Aelius facing straight onto the mausoleum – it still provides a scenic approach from the center of Rome and the left bank of the Tiber, and is renowned for the Baroque additions of statues of angels holding aloft instruments of the Passion of Christ.
Source: Wikipedia