The flat hilltop area behind the Fortezza was the original Etruscan acropolis of Velathri, the most important city of the northern Etruscan dodecapolis. The Parco Archeologico now occupies the site; the visible remains include the foundations of two temples (4th–2nd centuries BC) and a Roman cistern, with informative panels in Italian and English.
The park is also a pleasant green space — pine trees, benches, and an Enrico Fiumi memorial — and a good place to escape the lanes of the old town for half an hour. Combined ticket with the Roman Theatre and other municipal sites.