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You can open the gate, the instruction book gives a hint, but you need to have some prior knowledge about a certain secret classic gaming code...

I read that manual about 60 different times, and still couldn't understand what the up, up and down and down means. Is it like a cheat code or is it puzzle that I missed?

It's the Konami Code 👀^^ vv <> <> BA

Ooh, thank you so much! I finally beat the level, I didn't know the konami code existed back then xD