Castle on the Coast (EP)

In the Ocean part of the game (The Starting Point), find a circular structure underwater with a flow that pushes you down. Inside the cave, you should see a door with 9 on it. You need 9 glowing star thingie to open the door. Once inside, there will be a room with an arrow made of collectibles pointing down. Reach the bottom and you will land on the bottom of the ocean. Enter the door here, complete the platforming challenge to get the keystone.