Pull the lamp, land on Coastal Ruins.
Without this "secret" you can still continue the game: use the Levelgate to join Lost Library, return to Temple of Poseidon throught the crowlspace, and to Coastal Ruins from the trapdoor near the 2 Golden Bird "save-us-from-the demon" statues. A second possibility is from Temple of Poseidon to Catacombs, and to Coastal Ruins. Check my Levelgates.