This was fun and seems reasonably well balanced over time, which I think is a tricky line for these sorts of games to ride. I managed to play up to level 16 without crossing a threshold where the game didn't require my input in order to avoid eventual decline. Well done!
Playing with a keyboard, I found that the projectile angle not being aligned with grandma's sprite made it difficult to feel like I could aim effectively, but I suppose the genre de-emphasises the level of control I'm seeking.
While I enjoyed the music (and don't get tired of stuff I like), I can imagine that it may get repetitive for some players.
It's nice to see a DragonRuby game!