This is Circus Maximus. It is right behind Palatine Hill. Palatine Hill is where the Emperor (Dionysian, maybe??) built his Pallace. The Circus Maximus was right behind the Pallace, so the Emperor could watch events from his terrace. This is where the famous Chariot Races were held. (think Ben Hur) Hundreds of thousands of people could attend events here. It was much larger than the Colleseum.