Ehhh, I think you want a hint if I may believe Google translate. Spoiler: hint You need to get to the back of the windmill. There is a door on the side (opposite side of the waterwheel).
@Block builder : If you believe Google translate, you probably should have used it. Spoiler Anda perlu untuk mendapatkan ke belakang kincir angin. Ada sebuah pintu di sisi (sisi berlawanan dari kincir air)
oh i'm sorry for language.. finally i've solved it.. i was stuck for a few hour.. haha.. thanks for nice level @Block builder..
@chemi's comment gave me a helpful clue. Yes, a bit hidden and the reveal was a struggle to achieve, but worth it. I'm proud to have won, twice.