The round based temple of Vesta, with its 18 Corinthian order fluted columns (of which only 10 remain) and a peristyle covered by a travertine coffered ceiling, dates back to the first half of the 1st C. BC. and dominates the valley, now the site of the Villa Gregoriana.
This is the most renowned monument of the ancient Tibur and the elegance of its forms and its stunning setting have meant that it has become the symbol of the city, gloriously silhouetted against the sky on top of a rocky outcrop of the ancient acropolis.
The construction has a Corinthian peripteros of 14.25 metres in diameter set on a concrete podium 2.39 metres high covered in travertine arranged in opus quadratum. On top of this stands a round cell with a single door which tapers towards the top and two windows – of which only the right one is still open – surrounded by an ambulatory embellished by 18 Corinthian order columns topped by a travertine entablature decorated with alternating ox-heads and festoons.
In the Middle Ages the building was converted into a Church, as was the nearby Temple of the Sibyl, and called S. Maria Rotonda. It was the church of the local deaconship. The temple was later restored to its original structure, although traces of its conversion into a church still remain in a small niche of the cell where, at the beginning of our century, one could still make out fragments of Christian paintings: a central figure of a Virgin Mary, flanked by two saints.