Your game is very fun but I don't understand the connection to the theme. One suggestion (maybe i missed it) have a button to speed up the game. Sometimes it makes the game feel too slow watching the enemies make their way across the screen.
The connection to the theme was that the game is a puzzle game in disguise of a tower defense. I do admit that the connection isn't as strong as I would have liked. Though, I'm glad to hear that you enjoyed it. Also, thanks for the suggestion. You can reset all enabled keys to disabled with F2, but a button to speed up the game would be great addition.