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.

Michelangelo repurposed the ruins of Emperor Diocletian's baths into the church Santa Maria degli Angeli, where the massive scale of ancient structures is layered into a beautiful basilica. The Termini train station, named for "baths," is across the street, and Piazza della Repubblica, a common filming scene, is directly outside.