Pretty self explanatory.
On each island you will have two sides, one with a cave and other with a dock, with a greed generator just before the end of it.
When you destroy the violet thing, you can walk a bit and spend 6 coins to create a lighthouse. It will allow you to return to the island later without crashing your boat.
To get this achievement it does not matter which level you put coins on it. Just the 6 coins will do.
This is kind of missable since when you begin the game, on the first island you don't have knights to go and attack portals. You have to return later in the game travelling back with the proper building upgrades to be able to create it.
When you do that on the five lands / islands you will unlock it