The keyhole view of St. Peter’s is worth the walk to see it. However, the map on Trip Advisor for this is not correct. It is actually located south of Circo Massimo. In the middle of Circo Massimo take the street Via S. Sabrina towards Piazza Cavalieri di Malta. Once there look for the large green door #3 and then look through the keyhole. In total, it is only about a 15 minute walk from the Colosseum. There is a courtyard next to the church that you can go to and see a nice open view of Rome since you are up on Aventine Hill. There is also an orange grove and rose garden in this area. Very nice area to walk to.
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