The Aventine Keyhole - For those in the know, this is a popular attraction. Locals bring their visitors here to admire the impressive optical effect created by Piranesi. It doesn't sound that wonderful; some trees and St. Peter's, but few leave uncharmed. Located in the artistic and enigmatic Piazza dei Cavalieri di Malta, a gate bars the entrance to the Priory of the Knights of Malta. Queue up to have a look through the ornate keyhole, and you'll see the Vatican's vast white cathedral floating at the end of a symmetrical tunnel of trimmed trees.The piazza is placed on the Aventine Hill (Aventino), which is a nice tranquil place to stroll on a sunny day. To reach the keyhole, you follow Via di Santa Sabina from the Circo Massimo (Circus Maximus) past a rose garden, two attractive churches and a couple of delightful panoramic parks. One, a walled garden filled with orange trees, hosts outdoors theatre in the summer months.