Thanks so much for the feedback! You were right about running out of time on the art, it was the main reason as to why my ""beautiful"" programmer art made it into the final version of the game. I ended up spending about an hour making sounds, but I'm still pretty satisfied with how they turned out.
As for the levels, I hated myself for deciding to go with 100 levels while I was making this game, but I'm glad now and I'd say they turned out pretty well. The first 50 levels took forever to make because they were pretty much just teaching the player about the enemies. However, I was able to knock out the last 50 in 2 hours because they were all just amped up/harder versions of the first 50 levels.
I'm glad that the difficulty felt right and that you enjoyed my game!