Great game! It's such a simple concept but you were able to do a ton with it. I had a lot of good "ah-ha" moments when I was able to figure out puzzles in the game.
If you wanted to keep building, one thing you could do would be to add key pick ups that return a key to be used again. This could allow for some larger levels with more options available to the player.
Overall, fun game! Good luck with your future game dev!