This boundary was known as the Pomerium, and Romans thought it originally followed the line ploughed by Romulus around the city. But it wasn't just about marking out territory. Much more significantly, the Romans believed the land within it enjoyed divine protection. This gateway, known as the Arch of Dolabella, was built on the line of the Pomerium. And to keep the gods happy, there were strict rules dictating behaviour within Rome's sacred confines. So on this side of the arch, I'm outside the city, where the Romans believed you could bury your dead or make war. But as I walk through the archway, I enter Rome. Rome begins here, and everything is sacred. So if I was a soldier, for example, I would have to leave my arms outside the gate. Generals could not build military camps here. The Romans believed that to break these rules was sacrilege.