Instructions
Go inside the windmill and go up the left side of the gaming area. Move along to the right and when you reach the top, click on the rope attached to the lever. Jump to it and pull away the lever that engages the gears.
Go back outside and jump to the top of the door frame. Jump to one of the left blades to make the windmill blades spin counterclockwise.
Jump from blade to blade as they swing around. Leap onto the door frame again and jump back onto a left blade if you should accidentally fall back to earth. The hatch at the top of the windmill will slowly crank open and you will be able to enter.