Castle on the Coast (EP)

I'm unsure if this has a specified unlock requirement, or if it's simply for completing one of the levels.

Each point the game wants you to go to has a large aura around it. I simply followed them as a part of the story, and wound up completing the ocean level first and collecting the Ocean Keystone.

Upon leaving the level via the portal this achievement popped.
Most likely story related and cannot be missed.