This is a really fun idea! It's kind of funny how the towers look like... just towers, but they want to see the world.
I think some music would've been nice? There's a lot of good choices in opengameart if making music isn't your thing. That being said, I'm just personally biased towards music.
The standard towers are very cute! And it seems to run just fine, I feel like tower defense games have a lot more to do compared to other types of games, so it's impressive you managed to so much in such a short amount of time. Great job!