The Castel Sant'Angelo, or Mausoleum of Hadrian, is a large, circular structure on the Tiber River, visible from St. Peter's Basilica. Originally Hadrian's mausoleum, its solid construction made it an ideal papal fortress, connected by a visible passage to St. Peter's. It also served as a prison, holding Caterina Sforza and the fictional Mario Cavaradossi from Puccini's Tosca.