According to Livy, at a crucial moment that led to the Romans’ victory over the Latins at Lake Regillus, in the territory of Tusculum in 499 or 496 BC, the dictator Aulus Postumius:

1. “... is said to have vowed a temple to Castor ...”, (‘History of Rome’, 2: 20: 12).
   

In a later passage, he recorded that:

1. “The temple of Castor was dedicated on 15th of July [484 BC] . I t had been vowed during the Latin war by Postumius, the dictator. His son, having been made duovir for this special purpose [presumably after his death], dedicated it”, (‘History of Rome’, 2: 20: 12).
   

Dionysius of Halicarnassus gave a more elaborate account of these events:

1. “It is said that, during the battle at Lake Regillus, two men on horseback, far excelling ordinary humans in both beauty and stature and just growing their first beards, appeared to Postumius, the dictator, and to those arrayed about him, and charged at the head of the Roman horse, ... driving [the enemy] headlong before them.  Furthermore, after the flight of the Latins and the capture of their camp, ... two youths are said to have appeared in the same manner in the Roman Forum attired in military garb ... [with] the horses they led being all in a sweat.  When they had  watered their horses and washed them at the fountain which rises near the temple of Vesta, ... many people stood about them and inquired if they brought any news from the camp,.  They announced how the battle had gone and that the Romans were the victors.  It is said that, after they left the Forum, they were not seen again by anyone, though great search was made for them by the man who had been left in command of the city.  The next day, when those at the head of affairs received the letters from the dictator and ... learned of the appearance of the divinities, they presumably concluded ... that it was the same gods that  had appeared in both places, and were convinced that the apparitions had been those of Castor and Pollux”, (‘Roman Antiquities’,  6: 13: 1-3).
   

He then observed that:

1. “There are many monuments to this extraordinary and wonderful appearance of these gods at Rome:
   

2. ✴the temple of Castor and Pollux, which the city erected in the Forum at the place where their apparitions had been seen;
   

3. ✴the adjacent fountain, which bears the names of these gods and is to this day regarded as holy;
   

4. ✴the costly sacrifices that the people perform each year through their chief priests on [on 15th of July], the day on which they gained this victory; and
   

5. ✴the procession performed after the sacrifice by those who have a public horse and who, being arrayed by tribes and centuries, ride in regular ranks  on horseback, as if they came from battle, crowned with olive branches and attired in the purple robes with stripes of scarlet which they call trabeae.  They begin their procession from a certain temple of Mars built outside the walls, and going through several parts of the city and the Forum, they pass by the temple of Castor and Pollux, ... wearing whatever rewards for valour in battle they have received from their commanders ...
   

6. These are the things I have found both related and performed by the Romans in commemoration of the appearance of Castor and Pollux; and from these, as well as from many other important instances, one may judge how dear to the gods were the men of those times”, (‘Roman Antiquities’,  6: 13: 4-5).