3.5 miles, 1-3 hours
I want to share this walk because I'm deeply in love with the city of Rome. It's as beautiful at night as it is during daylight. Some parts are not that crowded (Saint Peters square and Colosseum) as they are during the day.
One tip: do not make this walk when your feet are already hurting from other walks you did during the day.
another tip: You should definitely take your camera for this trip. If you can, take a tripod as it's difficult to take good pictures by hand.
If you can, start the walk after dinner at night, when the sun is already gone.
Getting to the beginning of the walk is pretty easy: take the metro to the station of Ottaviano and walk to the Piazza San Pietro. You can also walk the other way, starting from the Colosseum, but not that you will be starting at the highlight of this trip! I think it's better to save the most beautiful part for the end. It can be quite tiresome to do the walk, so the Colosseum is a good goal for yourself as you DON'T want to miss this!