The Heroon, or circular temple, of which mention has been made, is similar to that near the Circus of Maxentius. The diameter of this is fifty-six feet, and it was lighted by four large round windows. The front was turned towards the road according to the rule laid down by the architect Vitruvius. Underneath, there is a crypt supported by a massive round pillar, and containing six niches. In this, Nibby thinks that the ashes of the dead were placed, as their statues were in the temple above, and that the building was the Heroon of the reigning family. In the middle ages this Heroon was used as a church, and some of the paintings then introduced are still visible on the interior walls. Not far from the Heroon are the ruins of the arcades which surrounded the stadium and bounded the domain of the villa on the east side. [Sidenote: Torre Pignatara.] In this district, but along the ancient Via Labicana which runs in the direction of Frascati, stands the conspicuous tower now called Torre Pignatara from its construction with pigne or earthen pots. It surmounts a large circular hall and a catacomb to which the titles of S. Helena’s Mausoleum and the Chapels of SS. Peter and Marcellinus have been given, but the real history of the building is unknown. [Sidenote: Ponte di Nono.] At the ninth milestone on the road to Palestrina, where the road crosses a small brook, is a magnificent monument of ancient Roman architecture, consisting of an arched viaduct built of peperino and tufa blocks. The length of this viaduct is 105 yards, and the highest of the seven arches about fifty feet. The blocks of stone used are in some cases ten feet in length, and they are firmly fitted together without any kind of cement. This viaduct is now called Ponte di Nono.[146] The ancient roadway of polygonal fragments of basalt still remains, but the parapet on each side has been destroyed. [Sidenote: Gabii.] At a distance of about three miles beyond the Ponte di Nono are the ruins of Gabii on the edge of the lake called Lago di Pantano in the district of Castiglione. Numerous traces of the ancient city are still visible. It occupied a long strip of ground extending from the sepulchral mound on the right of the road near the outlet of the lake to the tower of Castiglione. Nibby thinks that this tower stands on the spot formerly occupied by the citadel of Gabii, the original stronghold founded according to the legend by a colony from Alba. In the year 1792 extensive excavations were made on the site by Prince Marcantonio Borghese at the suggestion of Mr. Hamilton a Scotch painter, and a quantity of sculptures and inscriptions now in the Louvre at Paris were discovered. The principal ruins now remaining are those of the cella of a temple built of the famous lapis Gabinus, and some steps in a semicircular form, probably the remains of a theatre. The temple is generally supposed to have been that of Juno alluded to by Virgil.