There are really only two sites of interest in Tivoli (plus a third four miles out of town: the villa of Emperor Hadrian). The first is this, the Rocca Pia castle built by Pope Pius II. The castle is not tourable, so it's just a photo-op, but this is a burly monument to papal authority: Pius built it to quash the revolutionary thoughts of his restless population who lived in the surrounding neighborhoods.