The denarius was one of the most successful and long-lasting coins of the antique Mediterranean world. It was introduced around 211 BC and remained the major denomination of the Roman Empire for about 400 years.
The denarius showed recurring images on both sides: the obverse bore the head of Dea Roma and an X for the coin's value of 10 asses. The reverse represented the Dioscuri Castor and Pollux galloping into battle across the inscription ROMA.
The images and inscriptions on the denarii made it immediately clear to anybody living at that time that the coin had been issued by the Roman Republic. The state of Rome therewith assumed responsibility for the value of the coin.