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You can take in a lot of Rome in a day. Have you been before? If so, ignore most of what follows!The main ancient sights - the Forum, the Capitoline Hill, the Colosseum and the Palatine Hill - are all close together, and it's well worth strolling round all of them even if you don't go in. If you are going in, get a ticket at the Palatine Hill that allows you into the Colosseum and then go there immediately and go to the Palatine ruins afterwards (where you get great views, including of the Circus Maximus). The arms and armour in the Castel Sant'Angelo are worth a look, although it's a fairly small exhibition. But you could easily look round the ancient stuff in the morning and then stroll up along and across the Tiber to the castle in the afternoon. That would allow you to look into St Peters too. Most of the museums focus on ancient stuff or art rather than arms and armour (the Museo Stibbert in Florence is the motherlode for that ...).You could see fit in quite a few of the Baroque sights (Piazza Navona, etc.) too in the same day. Rome's a great city to walk around and easy to keep oriented in because the river keeps you right.
Looking at this map..... Is it feasible to walk say, from the train station, through the Colosseo and Old Rome areas, across the river to the Vatican, and back again, stopping in a museum or two along the way? No idea of how big Rome is to be honest....
I would stick with St. Peters, the Sistine chapel and the Vatican museums.Nice and compact, and more than a day's worth of stuff to see.