From The Art and Popular Culture Encyclopedia
The Mausoleum of Hadrian, usually known as the Castel Sant'Angelo, is a towering cylindrical building in Rome, initially commissioned by the Roman Emperor Hadrian as a mausoleum for himself and his family. The building was later used as a fortress and castle, and is now a museum. It is also known for its "Corridor of the Grotesque".
The castle appeared in the film Roman Holiday in a scene taking place on barges on the river below.
In Puccini's opera, Tosca, the Castel is where the quarter's of Scarpia are, and where Cavadarossi is held prisoner, tortured and executed. At the end of the Opera, Tosca leaps to her death from it's battlements.
Scarpia's quarters and the place of torture is actually in the Palazzo Farnese. After murdering Scarpia in his private room at the Palazzo, Floria Tosca goes to the Castel Sant' Angelo, safe conducts in hand, where her lover, Mario Cavaradossi is to be executed. She has been led to believe it will be a mock execution and is horrified to find her lover dead. Rather than be arrested by Scarpia's henchmen, she throws herself from the rooftop.