Castor was a great soldier and horse tamer and competed in many Olympic Games and won many of them. Castor was worshipped as gods by other athletes. Both were Argonauts on Jason's quest for the golden fleece.
When Castor and Polydeuces returned from Jason's quest, they became involved in a fight between two young men. A terrible battle followed and Castor was killed. Pollux was granted immortality by Zeus, but he persuaded Zeus to allow him to share the gift with Castor. As a result, the two spend alternate days on Olympus (as gods) and in Hades (as deceased mortals). See Dioscuri