But few traces of the ancient walls of the city are left. Nibby is however doubtless right in saying that there can be no question about their course along the northern and eastern sides of the city, where the brow of the hill is steep and perfectly adapted for defence by a wall placed on the edge of the rocky valley of the Anio. The citadel was probably situated in the quarter called Castro Vetere--where the two temples commonly called the temples of the Sibyl and of Drusilla stand, for it is plain that some pains have been taken to isolate this from the remainder of the site. On the western side the limit of the ancient walls is marked by the old gate, and by the fragments of walls which still exist at the point where the direct road from Rome enters the city by the modern Porta del Colle. The course of the walls then excludes the Villa d’Este, and runs across the hill to the Church of the Annunziata and the Porta Santa Croce and the citadel built by Pius II. on the site of the ancient amphitheatre. From thence the walls passed in a straight line down to the river near the Church of S. Bartolommeo. The ancient town did not extend to the right bank of the Anio. [Illustration: Temple of Vesta at Tibur.] [Sidenote: Temple of Vesta.] Two ancient temples are still standing in tolerable preservation at Tibur. The first of these is a small round temple perched on the very edge of the precipitous ravine through which the Anio dashes. It has been protected against the violence of the furious torrent by massive substructions, which apparently existed in ancient times and have often been renewed. Ten of the eighteen columns which formerly surrounded the inner chamber still remain. The details of this temple are rather peculiar in style, and show an originality of invention very rare in Roman architecture. The columns have Attic bases, but the grooves of the fluting are cut in a style which is neither Doric nor Ionic. They terminate above in an abrupt horizontal line, and reach at the foot of the column quite down to the base without any intermediate cylinder. The capitals exhibit a fantastic variety of the Corinthian order, having the second row of acanthus leaves nearly hidden behind the first, and a lotus blossom as the decoration of the abacus. The frieze is ornamented with the skulls of oxen and festoons, in the loops of which are rosettes and pateræ placed alternately. The inner chamber, which is built of opus incertum, is partly destroyed, but the lower half of the door and a window still remain.