God Damn The Garden

A little later into the game you'll run into a Carafield in a room with a bunch of white pillars. It'll warn you about the next area and talk about how it's dangerous because of the Spider King.

After defeating the Spider King, the carafield will give you permission to jump on it's back. Jump up on the white pillars and look for a corner with a torch.

Jump across the pillars until you reach the torch and then immediately to your left is a path to the secret area.

There's also a golden skull at the end of this secret area!