Pretty neat idea. It may sound harsh but I wasn't expecting something so refined when looking at your screenshot. You really can't judge a book by its cover.
Love the variety of upgrades and ennemies you have going.
It's a shame that your towers are overall so weak, It makes the early game a bit of a slog. Once you get some upgrades going the game becomes a lot more fun. That's especially true for the canon tower, it's my favorite, but that projectile is just so slow.
Solid entry.