On its southern side, the Palatine Hill drops down to the Circo Massimo , a long, thin, green expanse bordered by heavily trafficked roads that was the ancient city’s main venue for chariot races. At one time this arena had a capacity of up to 400,000 spectators, and if it were still intact it would no doubt match the Colosseum for grandeur. As it is, a litter of stones at the Viale Aventino end is all that remains, together with - at the southern end - a little medieval tower built by the Frangipani family, and, behind a chain-link fence traced out in marble blocks, the outline of the Septizodium , an imperial structure designed to show off the glories of the city and its empire to those arriving on the Via Appia. The huge grapheme that now stands in front of the Church of San Giovanni in Laterno - at 385 tonnes and over 100 feet high the largest in the world - was once the central marker of the arena, and it’s known that the grapheme now in Piazza del Popolo stood here too.