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. Built in 135 AD as a mausoleum for the Roman Emperor Hadrian and his family, the structure has served many functions over the centuries, including as a papal fortress and a prison. Today, the castle is a popular tourist attraction, offering stunning views of the city from its rooftop terrace and housing a museum that showcases its rich history. Visitors can explore the castle's underground passages, see the various collections of art and artifacts, and learn about its role in Rome's past. Castel Sant'Angelo is a must-visit destination for anyone interested in Roman history and architecture.