There are three bus lines that stop few steps from the apartment and get to St. Peter's and the Vatican (you can also walk for about 20 minutes to get there). One of them, in particular, gets to Piazza Venezia, the heart of the historical centre, and close to the Colosseum and Foro Romano. It passes near Piazza Navona and Pantheon, as well. There are other three bus lines that get to the nearest metro station in a couple of stops (but you can also walk for about 10 minutes to reach the metro station).