Overall a really cool, very finished and polished game! Thanks a lot for making it, I had a great time playing! :)
I loved the graphics, especially the reward at the end, for an artist like me, clearly it was the best reward to see a pretty pixel art sprite x) Everything fits well together, and especially I found that the skulls looked very cool.
I also loved the sound design, fits the mood very well, and the voice acting is a really nice touch.
The levels are nice, I'm not much of a puzzle person so often when I play them I just the move the tiles halfheartedly until it fits, but in this game I ended up making strategies and thinking about constraints, so great job! Especially I loved that some of the last levels were 3x3, it's always nice when complexity arises from very simple-looking problems (like a "oh this is easy... oh wait" feeling).
Just one downside, it was hard for me to get used to the controls. I often wanted to just click on two tiles to exchange them, so the fact that the tile stayed selected until re-clicked made me do mistakes in the beginning. I also think this "move one tile at a time" thing makes you believe as a player that the movement is going to be a constraint (like in a fifteen puzzle), when it's the positions that are constraints. But maybe the game feel is still better like that, because it adds more physicality, it's just a thought I had while playing.
Amazing job!