Aventine Keyhole

There is a green door on the Aventine Hill, at the top of a quiet, orange-tree-scented lane, and if you put your eye to the keyhole you will see one of the most perfectly composed views in the world. Through the keyhole, framed by a tunnel of precisely trimmed hedges, you see the dome of St Peter's Basilica floating at the end of a green corridor. It is the only place in Rome where you can see three sovereign states at once: Italy (where you are standing), the Priory of the Knights of Malta (the garden beyond the door), and Vatican City (the dome in the distance).

The Priory belongs to the Sovereign Military Order of Malta, which is technically its own sovereign entity — it issues passports, has diplomatic relations with over a hundred countries, and has permanent observer status at the United Nations, despite having no actual territory beyond this compound and a palazzo elsewhere in Rome. It is the world's smallest sovereign entity by population, with about thirteen thousand five hundred members. The gate was designed by Giovanni Battista Piranesi, the same artist famous for his etchings of fantastical prisons.

The garden behind the door is not open to the public, but the keyhole view has been a Roman insiders' secret for centuries. Nobody is entirely sure if the alignment of the hedges to perfectly frame the dome was intentional from the start or a happy accident that was later cultivated. Either way, it has become one of those rare tourist attractions that actually exceeds expectations — the compression of the telephoto-like view through a tiny aperture makes the dome look impossibly close and impossibly perfect.

There is usually a line. It moves fast, because everyone takes about five seconds — you look, you gasp, you step aside. The piazza itself, with its Renaissance walls decorated by Piranesi, is one of the quietest spots in central Rome. Come at sunset when the light hits the dome gold.