I really love that you made a one button game!
But while you prevented the player from staying at the bottom of the latter the avoid the asteroid you forgot to add a barrier at the top. You can literally just hold space and never loose.
But aside from that bug the game is great. To improve it, you could add more upgrades to the towers. Maybe introduce a second dimension but as I said, I love the one button idea, so think twice before you add another button.