The Mausoleum of Hadrian, also known as the Castel Sant'Angelo, is a cylindrical structure located in Rome, Italy. It was originally built as a tomb for the Roman Emperor Hadrian and his family around 135 AD. The mausoleum was later used by the popes as a fortress and residence, showcasing its historical significance over the centuries. The structure stands approximately 64 meters tall and is adorned with a statue of an angel on top, which gives it its current name. The Mausoleum of Hadrian is an important example of ancient Roman architecture and is a popular tourist attraction, reflecting the rich history of Rome.