Its charming Old Town, as well as having interesting streets and squares, features a number of vantage points from which you can see beautiful views of the valley and the foothills of the city of Rome. Another noteworthy feature of this small village are the ancient cyclopean walls of pre-Roman origin, present in one part of the old town.
Here's a charming little video, which offers more scenes of the town. It's in Italian: basically they talk about the myth that Saint Peter came to preach in the town, and how the town was made famous as the setting for several movies starring Vittorio de Sica and Gina Lollobrigida.
Castel San Pietro Romano