What is "regillus"

Regillus was an ancient lake of Latium, Italy, famous in the legendary history of Rome as the lake in the neighborhood of which occurred (496 B.C.) the battle which finally decided the hegemony of Rome in Latium. During the battle, so runs the story, the dictator Postumius vowed a temple to Castor and Pollux, who were specially venerated in Tusculum, the chief city of the Latins (it being a Roman usage to invoke the aid of the gods of the enemy), who appeared during the battle, and brought the news of the victory to Rome, watering their horses at the spring of Juturna, close to which their temple in the Forum was erected. There can be little doubt that the lake actually existed. Of the various identifications proposed, the best is that of Prof. L. Pareti, who finds it in a now dry crater lake ( Prataporci), drained by an emissarium, the date of which is uncertain, north of Frascati. Most of the other sites proposed are not, as Regillus should be, within the limits of the territory of Tusculum.