Poseidon and Apollo, having offended Zeus, were sent to serve Laomedon. He had them build huge walls around the city. In this they were aided by Aeacus, king of Aegina. When they had finished, three serpents tried to destroy the walls. The two attacking Poseidon and Apollo's sections died instantly but one forced its way through Aeacus' part. Apollo prophesised Troy could only be breached by the Aeacidae; Aeacus' descendants.