I enjoy these single tower defense-type games more than placing multiple towers and trying to juggle a dozen things at once. You did a great job on this.
Adding some hit sprite animation effects would be nice. I think if you make it so the player can hold down their button to attack the enemy continuously that'd save a lot of the tiredness the player will feel. I think you can make the upgrade menu slow down the game to half speed and then introduce power-ups to increase that pause length would be cool.
Also, when it gets more difficult, I'd like to see automated or semi-automated turret power-ups.
Keep up the good work.