Castel Sant'Angelo, also known as the Mausoleum of Hadrian, is a historic building that was originally built as a tomb for Emperor Hadrian and his family. It later served as a fortress, residence, and prison for the popes. Legend has it that the Archangel Michael appeared atop the mausoleum, giving it its current name. Today, it's a museum where visitors can explore its rich history, including its use as a prison for notable figures like Giordano Bruno and Benvenuto Cellini. With stunning views of Rome from the top, Castel Sant'Angelo offers a fascinating glimpse into the city's past.