Aventine Keyhole

On the quiet summit of the Aventine Hill, the green door of the Priory of the Knights of Malta conceals Rome's most famous peephole. Press your eye to the keyhole and a perfectly framed view appears: a long hedge-lined corridor draws your gaze through to the dome of St. Peter's Basilica floating in the distance, centred with geometrical precision. The optical trick — designed by Piranesi in the 18th century — compresses three sovereign territories into one sight line (Italy, the Knights of Malta's extraterritorial priory, and Vatican City). The queue can build on weekends, but move quickly. It's a 30-second experience that photographs beautifully and never fails to delight.