Where to find the "keyhole" view in Rome?
 

We have heard of a "keyhole" view (I believe of the Vatican) that can be found in Rome. Friends of ours were taken there by taxi but had no idea where they were. Can anyone give me specific directions to find this keyhole and confirm what can be seen through it?

The keyhole is in the green gate of number 3 at Piazza dei Cavallieri di Malta on the Aventine Hill. You can see a framed view of the dome of St. Peter´s.