Castor was mortal, but his brother Pollux was immortal; so when the former was slain in fight Pollux obtained from Jupiter permission that each should spend half their time in heaven, half in Hades.

Theseus descended into Hades in order to carry off Proserpine.

He was kept a prisoner there until he was rescued by Hercules (Alcides), who came down to carry off Cerberus, the three-headed dog who guarded the entrance (see stanza lvi.).
XXXII.

Virgil alludes to the promontory of Misenum on the north side of the bay of Naples.

The legend is a purely local one.