Castel Sant'Angelo, also known as the Mausoleum of Hadrian, is a historic fortress located on the banks of the Tiber River in Rome, Italy. Originally built by the Roman Emperor Hadrian in the 2nd century as a mausoleum for himself and his family, it has served various purposes over the centuries, including a papal residence, military stronghold, and prison. The castle is renowned for its striking cylindrical design and the angel statue on top, which gave it its current name. Today, Castel Sant'Angelo houses a museum and offers panoramic views of Rome.