Italy - 12 hour layover in Rome

I am planning a 12 hour layover (9am to 9pm) in Nov or Dec. and would like some advice on a touring plan for the city that is reasonable and doable.

I have never been to Rome and am streching the necessity of a connecting flight to get in as much sightseeing as possible

Thanks in advance

I am planning a 12 hour layover (9am to 9pm) in Nov or Dec. and would like some advice on a touring plan for the city that is reasonable and doable.

I have never been to Rome and am streching the necessity of a connecting flight to get in as much sightseeing as possible

Thanks in advance

Get the train FR1 from FCO to 'Roma Ostiense', then take the underground line 'B' to 'Colosseo'. You'll reach the ancient area of Rome, where a number of attractions are: Colosseo, Fori, Circo Massimo, Palatino, Campidoglio, Altare della Patria, Via del Corso, Fontana di Trevi, Piazza di Spagna, Piazza del Popolo, Pantheon, Piazza Navona, Ghetto and Isola Tiberina, etc.
Just take a walk and enjoy.