The keyhole

This green door has a keyhole that people have been peering through for two hundred and fifty years, and what you see is one of the most carefully staged views in the city.

Through the hole you look into a garden where a hedge of cypresses runs forward like a tunnel, and at the very end of that tunnel sits the dome of Saint Peter's, a kilometre and a half away. The hedge is clipped so the dome lands dead centre, and the whole effect comes from Piranesi's design for this site.

There's one more lovely detail hidden in it. You're looking from the territory of the Order of Malta, across Italian soil, towards Vatican ground, so three sovereign entities in a single glance.

The hole is small and the queue can build up, so early in the morning you'll find it quieter. The garden behind it can only be visited by appointment.

And with that you've come full circle. We take Via di Porta Lavernale, which runs down from the square, and after ten minutes we're back at the pyramid, with the Piramide metro stop right beside it.