Titans' Castle

The Titans' Castle is the final destination of the game, and can be found by summoning Poseidon's ship just outside the shrine where you fought Cronus, and submerging there. As long as you've fought Cronus, the gate to the castle will be open. The only treasure to get here is the Zeus Armor. Other than that, you need to defeat the four bosses, one in each of the four towers, before you can climb the central tower that leads to the game's final boss.