Temple of Castor and Pollux, the three remaining columns in the Forum Romanum, Rome. Temple of Antoninus Pius and Faustina and Basilica of Maxentius and Constantine in background

The Temple of Castor and Pollux (Italian: Tempio dei Dioscuri) is an ancient edifice in the Roman Forum, Rome, central Italy. It was originally built in gratitude for victory at the Battle of Lake Regillus (495 BC.). Castor and Pollux (Greek Polydeuces) were the Dioscuri, the "twins" of Gemini, the twin sons of Zeus (Jupiter) and Leda. Their cult came to Rome from Greece via Magna Graecia and the Greek culture of Southern Italy.
The octastyle temple was peripteral, with eight Corinthian columns at the short sides and eleven on the long sides. There was a single cella paved with mosaics. The podium measures 32 m × 49.5 m (105 ft × 162 ft) and 7 m (23 ft) in height. The building was constructed in opus caementicium and originally covered with slabs of tuff which were later removed. The temple was probably already falling apart in the fourth century, when a wall in front of the Lacus Juturnae was erected from reused material. Nothing is known of its subsequent history, except that in the 15th century, only three columns of its original structure were still standing. The street running by the building was called via Trium Columnarum.
The last king of Rome, Lucius Tarquinius Superbus, and his allies, the Latins, waged war on the infant Roman Republic. Before the battle, the Roman dictator Aulus Postumius Albus Regillensis vowed to build a temple to the Dioscuri if Rome were victorious.
According to legend Castor and Pollux appeared on the battlefield as two able horsemen in aid of the Romans. And, after the battle had been won they again appeared on the Forum in Rome watering their horses at the Spring of Juturna thereby announcing the victory. The temple stands on the supposed spot of their appearance.
In Republican times the temple served as a meeting place for the Roman Senate, and from the middle of the 2nd century BC the front of the podium served as a speaker's platform. During the imperial period the temple housed the office for weights and measures, and was a depository for the State treasury.

more on https://en.wikipedia.org/wiki/Temple_of_Castor_and_Pollux , https://en.wikipedia.org/wiki/Temple_of_Antoninus_and_Faustina and https://en.wikipedia.org/wiki/Basilica_of_Maxentius