Thanks for the detailed feedback, Bananapigs! :D
I see where you're coming from when it comes to level progression, now that you mention it. The first 10 levels are meant to teach the player to avoid vases, go quickly, grab coins, snag the diamond and run for the exit, but I probably could have taught those mechanics a little quicker and gone straight to the fun stuff right away. The fun stuff being slugs and turrets, of course. ;P
That's definitely a result of creating the levels as I went along, instead of making a proper design document before starting work on the code. By the time I had incorporated slugs and turrets, which contribute the most to speedrunning-goodness and the challenge of the game, I'd already created most of the levels, and (perhaps wrongly) decided to keep the game progression the same to avoid having to recreate all the levels.
Next time I make a game which relies so heavily on enemy behavior, I'll be sure to create the objects, try them out with players in a playtesting environment, and build levels from the ground up based on what works.
As for your feedback about music, I tend to agree as well. It's a tough balance to strike when it comes to the amount of music in a game. Too much becomes repetitive, but not enough makes the game feel empty. I'm yet to hit that balance, but I'll do my best to include a little more music in my next game!
Thanks again for playing, and I really do appreciate the feedback! :)
PS: If you beat level 30, you'll find that Bandit Mode might add a little more challenge to those early levels. ;)