Definition: The Circus Maximus (lit. 'biggest circus') was the first and the biggest circus in Ancient Rome.
"The installation of a circus on this spot was due to the first Etruscan king of Rome, Tarquinius Priscus. According to legend, the races were initiated by Romulus on the occasion of the festival that ended in the rape of the Sabine women."In ancient Rome spectators went to the Circus Maximus to watch chariot races. The shape of the circus was like an oval with a flattened end which was well-suited for the circuit the charioteers made. Although other events were held in the arena of the circus, the colosseum, with its round shape and more circular seating, was preferred for gladiator and wild beast combats. When Pompey the Great put on an elephant fight in the Circus Maximus, the elephants tore down the bars protecting the spectators. Pompey's contemporary Julius Caesar put a protective moat in the Circus Maximus, which the emperor Nero disposed of.
Source: The Vallis Murcia: The Circus Maximus (http://www.unicaen.fr/rome/anglais/geographique/maximus.html)