the Key can unlock the door. After obtaining a key, you can use it by going to the keyhole, you can try a few more times or give the PC version a shot.
Alright, I've updated and uploaded a new version of the game. It runs fine on my own computer, which puzzles me, but thank you very much for playing and trying it! (Once you obtain the key and use it on the keyhole, the door at the cave entrance will open, and escaping through the cave will complete the level!)