I just beat it again to get the badge, and even though I knew how to do everything this time through, it was still wonderful and charming. Thank you for this game!
It would be really nice to have a practice mode for some of the later levels that doesn't count towards the badges, but doesn't require you to go all the way through the beginning levels.
The key to winning hard is using the tacks to break up the black bloons. Using that strategy with lots of bomb towers and tack towers at the U-turns and all four corners of the crossing should get you through hard without too much trouble.