Amazing game! I thoroughly enjoyed playing it! My only real complaint is the immersion-break at the end with the credits. It would've been great if it had kept the palette- and also had an easy in-game way to restart the game. I wanted to play again immediately but had to refresh the page to do so. In the downloaded version, that's not even an option, you have to close and restart the application, which just doesn't feel good.
In addition, although it's a minor nitpick, I noticed that the animation of the chest at the beginning (although it looks great) squashes the sprite in a way that wouldn't be possible on a Game Boy.
All that aside, the fact that my biggest complaint is I can't immediately start playing again more readily is proof enough that this is a great game. I honestly wanted to keep going and was disappointed it ended when it did. I'd love to see an expanded game with more upgrades and enemies.
By the way, I couldn't help but noticed that the player has a wall-slide animation, but does nothing with it. Planned feature that didn't pan out due to time-constraints, I imagine? Perhaps there was going to be wall-jumping? I can just imagine actually trying to climb up the room to reach higher levels, maybe with new merchants/upgrades available when you reach them, but also new enemies.
But I'm rambling. Summary: I love the game!