The fountain at the end of a Roman pipe

You hear the Trevi before you see it, because it's stuck against the back of an ordinary apartment block on a street that's far too narrow for it. That's the surprise, really: a facade thirty metres wide down an alley you almost stumble into by accident.

The water behind it is older than anything you can see. The Acqua Vergine was built in 19 BC, runs mostly underground, and because of that it was the only Roman aqueduct to survive centuries of sieges. Enemy armies loved to tear aqueducts down, and they left alone what they couldn't see.

Up on the right there's a relief almost nobody looks at, because the horses and the sea god in the centre take all the attention. It shows a young girl pointing thirsty soldiers towards the spring, the story that gave the pipe its name.

Over on the right, where the balustrade begins, stands a large stone vase that carries nothing. A barber in the house behind it is said to have complained loudly about the construction every day, so the architect had the thing placed right in his line of sight. Romans still call it the ace of cups.

We take Via delle Muratte straight across from the fountain, cross the busy Via del Corso, and keep going until we're standing in front of a church with a wide little flight of steps.