I looked through all the posts and didn't see my favorite site: The Priory of The Knights of Malta, a doorway with a keyhole that looks across to St.Peters Basilica. I think it's also called The Aventine Keyhole. I saw it in a magazine when I was young and had the photo framed on my wall. Being there and seeing it was a dream come true for me.You can walk all the way up there, but you can also hire a car or taxi: atlasobscura.com.