Helped by Hermes, Athena, and some fresh-water nymphs, who collectively provided him with a magical sword, shield, helmet of invisibility, winged sandals, shoulder bag and advice, Perseus succeeded in slaying Medusa because he knew he could look at her reflected in his shiny shield, and know where to aim the killing blow. On his way back from this adventure, he discovered the lovely Libyan princess Andromeda who had been chained to a rock awaiting death from a whale-like sea monster, Cetus. He saved her (remember, he's a hero!) and married her. Libyan princesses figure frequently in Greek myth - Io and Europa were also believed to be from the coast of Libya, which was just far enough away to be exotic to the Greeks.