Lake Regillus, a small body of water in Latium, where, according to the historical legends of ancient Rome, was fought the battle which decided the fate of the last Roman king (about 498 B. C). Its site has been considered doubtful, but it is now generally supposed to be identical with the lake of Cornu-felle, at the foot of the hill on which stands the town of Frascati (the ancient Tusculum), about 10 m. S. E. of Rome; this lake was formed by a volcanic crater about half a mile in diameter, and was artificially drained in the 17th century. Here Tarquin the Proud, having after the failure of Porsena's expedition enlisted the Latins in his cause, with his son-in-law Octavius Mamilius gave battle to the Romans under the dictator Albinus Postumius, was wounded and utterly defeated, and fled alone from the field. The chiefs on both sides were nearly all killed or wounded. According to the tradition, the Romans owed their victory to Castor and Pollux, who were seen to charge at the head of their cavalry, and who first made it known in the forum at Rome. There a temple was dedicated to them, where the day (July 15) was afterward annually celebrated.