Vowed in hope of victory at the legendary Battle of Lake Regillus, where the Dioscuri appeared after the battle. Dedicated in 484 BC, restored in 117 BC, rebuilt by Tiberius (who rededicated it in AD 6) with the spoils of the German campaign (Suetonius, XX), and incorporated by Caligula as the vestibule of his palace and the Dioscuri its gate-keepers (Suetonius, Life, XXII.2; Dio LIX.28.5), the Temple of Castor and Pollux frequently served as a meeting place for the Senate and was where the standards of weights and measures were kept. There was a high podium, with stairs leading down to a rostra and, in the intercolumniations at the base, a series of chambers (loculi). Originally, the front of the podium was designed to provide a tribunal or platform for speaking; but, by the early third century, the lateral stairs had been replaced by a single flight in front. Clodius completely demolished the steps, effectively transforming it into a fortress.
All that now remains are three graceful Corinthian columns, still surmounted by an entablature with a plain frieze and modillion cornice. They are one of the most recognized landmarks of the Forum.
"When aedile, Caesar decorated not only the Comitium and the Forum with its adjacent basilicas, but the Capitol as well, building temporary colonnades for the display of a part of his material. He exhibited combats with wild beasts and stage-plays too, both with his colleague and independently. The result was that Caesar alone took all the credit even for what they spent in common, and his colleague Marcus Bibulus openly said that his was the fate of Pollux: 'For,' said he, 'just as the temple erected in the Forum to the twin brethren, bears only the name of Castor, so the joint liberality of Caesar and myself is credited to Caesar alone'" (Suetonius, Life, X.1).
"It is said that in this battle two men on horseback, far excelling in both beauty and stature those our human stock produces, and just growing their first beard, appeared to Postumius, the dictator, and to those arrayed about him, and charged at the head of the Roman horse, striking with their spears all the Latins they encountered and driving them headlong before them. And after the flight of the Latins and the capture of their camp, the battle having come to an end in the late afternoon, two youths are said to have appeared in the same manner in the Roman Forum attired in military garb, very tall and beautiful and of the same age, themselves retaining on their countenances as having come from a battle, the look of combatants, and the horses they led being all in a sweat. And when they had each of them watered their horses and washed them at the fountain which rises near the temple of Vesta and forms a small but deep pool, and many people stood about them and inquired if they brought any news from the camp, they related how the battle had gone and that the Romans were the victors. And 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 besides the other particulars of the battle, learned also of the appearance of the divinities, they concluded, as we may reasonably infer, that it was the same gods who had appeared in both places, and were convinced that the apparitions had been those of Castor and Pollux. Of this extraordinary and wonderful appearance of these gods there are many monuments at Rome, not only the temple of Castor and Pollux which the city erected in the Forum at the place where their apparitions had been seen, and the adjacent fountain, which bears the names of these gods and is to this day regarded as holy, but also the costly sacrifices which the people perform each year through their chief priests in the month called Quintilis, on the day known as the Ides, the day on which they gained this victory. But above all these things there is 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, sometimes to the number even of five thousand, wearing whatever rewards for valour in battle they have received from their commanders, a fine sight and worthy of the greatness of the Roman dominion. 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."
Dionysius of Halicarnassus, Roman Antiquities (V.13.1-5)