Simple fun.
I only made it to level 19, but I defiantly would be playing and probably will be playing more tomorrow. The auto save feature is really nice. Simple games like this show that core game dynamics can make a fun and addicting experience.
Hard levels made defeating them oh so much sweeter. I would recommend a bit more thought into the level placement. Some levels seemed way too easy to be in the higher tiers. Also add more music if possible, but if you want to keep the file size down then I guess you did the right thing by sticking to one simple loop.
I look forward to your future projects!