Hidden Island 15 shares Haunted Ship's design. The area is a gloomy nautical graveyard that is covered in perpetual thick, purple fog which obscures the sky and horizon. Rotting, half-sunken and abandoned sailing ships with tattered sails litter the waters, the majority of them having drifted together to practically form an island. Overall, this area has a spooky atmosphere that suggests it is haunted.
Hidden Island 15 is located southeast of Sky Babylon, in the lower right corner of the game's Sea Chart.
Like most other Hidden Islands, Hidden Island 15 is a short stage which can be finished in less than a minute. Borrowing elements exclusive to Haunted Ship, the main gimmicks here are blue ropes that the player can bounce on and use Trick Actions on. The player can jump higher by landing hard on them or by jumping on them several times in a row. Then, there are skeleton barrels hanging from ropes. If the characters get below them, they will drop down, capture them and go back into the air. However, the player can break free by repeatedly pressing . While sometimes a hindrance, they can also be used to reach higher areas.
In one spot, there is also a breakable barrel that contains a Spring.
At the beginning, follow the path until the characters hit a Spring leading to a ledge with a barrel. Break that barrel to unveil a Spring and use it to reach the platform above. Cross the following series of platforms and ropes to the left. At the end, use a skeleton barrel to reach the series of platforms and ropes above. Continue down those platforms and ropes to the right until the Treasure Chest appears.