Peek through the keyhole of the Priorato di Malta, the walled compound of the Knights of Malta, and you'll get a surprising eyeful: a picture-perfect view of the dome of St. Peter's Basilica, far across the city. Giovanni Battista Piranesi, 18th-century Rome's foremost engraver, is more famous for rendering architecture than for realizing it, yet the square is his work along with the priory (1765) within. Stone insignia of the Knights notwithstanding, the square's most famed feature is that initially nondescript keyhole in the dark green door of No. 3. Bend slightly and surprise your eyes with a view that is worth walking miles for. As for the Order of the Knights of Malta, it is the world's oldest and most exclusive order of chivalry, founded in the Holy Land during the Crusades. Though nominally ministering to the sick in those early days—a role that has since become the order's raison d'être—the knights amassed huge tracts of land in the Middle East. From 1530 they were based on the Mediterranean island of Malta, but in 1798 Napoléon expelled them and, in 1834, they established themselves in Rome. Tours are sometimes available if you would like to go inside; call for information.