In ancient Rome, chariot races commonly took place in a circus. The main centre of chariot racing was the Circus Maximus in the valley between Palatine Hill and Aventine Hill, which could seat 250,000 people. It was the earliest circus in the city of Rome. The Circus supposedly dated to the city's earliest times, but Julius Caesar rebuilt it around 50 BC to a length and width of about and , respectively. One end of the track was more open than the other, as this was where the chariots lined up to begin the race. The Romans used a series of gates known as "carceres", equivalent to the Greek "hysplex". These were staggered like the "hysplex", but in a slightly different manner since the center of Roman racing tracks also included medians (the "spinae"). The "carceres" took up the angled end of the track, where – before a race – the chariots were loaded behind spring-loaded gates. Typically, when the chariots were ready the emperor (or whoever was hosting the races, if outside of Rome) dropped a cloth known as a "mappa", signalling the beginning of the race. The gates would spring open at the same time, allowing a fair start for all participants.