Castel Sant'Angelo, also known as the Mausoleum of Hadrian, is a historic fortress located in Rome, Italy. It was built between 135 AD and 139 AD by the Roman Emperor Hadrian as a mausoleum for himself and his family.

The structure has undergone various transformations throughout history. In the Middle Ages, it was converted into a fortress by the popes, and it was also used as a residence and prison. In the 16th and 17th centuries, the castle was renovated and expanded by various popes, including Pope Paul III and Pope Clement VII.

The castle is located on the right bank of the Tiber River, near the Vatican City. It consists of a circular central building, surrounded by four bastions, and it is connected to the nearby Vatican City by a fortified corridor called the Passetto di Borgo.

Today, Castel Sant'Angelo is a museum that showcases a collection of artworks, weapons, and military memorabilia. Visitors can explore the castle's interior and climb to the top of the ramparts for stunning views of Rome. It is a popular tourist attraction in the city and is often featured in films and television shows set in Rome.