This is one of the strangest and most impressive sights in Rome: a Renaissance palace - still inhabited, though now divided up into apartments - grafted on to an ancient, time-worn theatre. Julius Caesar began building a massive theatre here to rival Pompey's in the Campus martius, but it was finished in 11 BC by Augustus, who named it after his favourite nephew. At one time it was connected to the Portico d'Ottavia, and originally had three tiers in different classical styles (Ionic, Doric and Corinthian): the top one has collapsed. Its 41 arches were topped with great marble theatrical masks. It seated up to 20,000 people.
After the theatre was abandoned in the fourth century AD it had various uses: at the end of the 12th century the powerful Savelli clan turned it into the family fortress; in the 16th century they commissioned Baldassare Peruzzi to convert it into a less defensive-looking family pile.
Now known as the Palazzo Orsini (little bear… stone interpretations of which can be seen on gateposts and elsewhere) after another owner, it has been split up into luxury apartments. The shops that filled the ground-level archways were thrown out in the 1930s, leaving the structure in splendid if rather sad isolation, its travertine blocks looking rather like they're suffering from some wasting disease. Get a close-up look from the walkway below the Portico d'Ottavia.