Aventine Keyhole

The green door on Piazza dei Cavalieri di Malta leads into the villa of the Priory of the Order of Malta, which has extraterritorial status. Look through the keyhole and, at the end of a hedge-lined avenue, St Peter's dome sits perfectly framed: three states at a glance, Malta, Italy and the Vatican.

The square and the door were designed in 1765 by Giovanni Battista Piranesi, the engraver of the famous views of Rome, and they are the only architecture he ever built. The obelisks and trophies on the walls are his.