CIRCEIUS MONS (mod. MONTE CIRCEO), an isolated prom ontory on the south-west coast of Italy, about 8om. S.E. of Rome. It is a ridge of limestone about 32m. long by im. wide at the base, running from east to west and surrounded by the sea on all sides except the north. The land to the north of it is 53ft. above sea-level, while the summit of the promontory is I,775ft. The origin of the name is uncertain : it has naturally been con nected with the legend of Circe. It is true that the promontory ceased to be an island at a very early date ; but it looks exactly like one from a distance. Upon the east end of the ridge are the remains of an enceinte, forming a rectangle of about 200 by iooyds. of very fine polygonal work. It seems to have been an acropolis, and contains no traces of buildings, except for a sub terranean cistern, circular, with a beehive roof of converging blocks. The modern village of S. Felice Circeo occupies the site of the ancient town, the citadel of which stood on the mountain top, for its mediaeval walls rest upon ancient walls of Cyclopean work of less careful construction than those of the citadel, and enclosing an area of 200 by I5oyds.
Circeii was founded as a Roman colony at an early date. At the end of the republic, the city of Circeii was no longer at the east end of the promontory, but at the south end of the Lago di Paola (a lagoon—now a considerable fishery—separated from the sea by a line of sandhills and connected with it by a channel of Roman date; Strabo speaks of it as a small harbour) . The trans ference of the city did not, however, mean the abandonment of the east end of the promontory, on which stand the remains of several very large villas. An inscription, indeed, cut in the rock near S. Felice, speaks of this part of the promonturium Veneris (the only case of the use of this name) as belonging to the city of Circeii. For its villas Cicero compares it with Antium, and both Tiberius and Domitian possessed residences there. The villa of the latter indeed, on the east shores of the Lago di Paola, has been mistaken for the Roman town of Circeii. It extends over an area of some 600 by 5ooyds., consisting of fine buildings along the lagoon, including a large open' piscine or basin, surrounded by a double portico, while farther inland are several very large and well-preserved water-reservoirs, supplied by an aqueduct of which traces may still be seen. The beetroot and oysters of Circeii had a certain reputation.
The view from the highest summit of the promontory (which is occupied by ruins of a platform attributed with great prob ability to a temple of Venus or Circe) is of remarkable beauty; the whole mountain is covered with fragrant shrubs. From any point in the Pomptine Marshes or on the coast-line of Latium the Circeian promontory dominates the landscape in the most remark able way.
See G. Lugli, Forma Italiae i. i. 2. (1928), the Italian archaeological survey.