Fun little game!
I was a bit lost until I read the comments as to what to do though. The visual indication isn't very clear as to the buttons being pressed and actually had pressed one without even realizing it. So it would help a lot to have extra visual indication than just the button changing slightly, as well as sound effects for the button presses as well as the gate opening so it's clear when you do things. Two sites that are good for free sound assets like that are pixabay and opengameart.org.
It would also help to have a description on the page that says the controls and objective of the game.