Go to the front side of the topmost (4th) deck. You will find two sloped ledges there. You can use either, but for this walkthrough we will use the right one. Stand against the back-side of the sloped ledge and jump up to grab it.

Shimmy to the very left.

Pull up and you will start sliding. Halfway down, press the Jump button along with the left direction key to grab the edge of the roof.

Pull up and head forwards. Jump to the ledge and fall through the trapdoor.