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.
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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.