Good game, but needs improvements:
I liked the graphics, the setup, and the variety. The sound works well, doesn't annoy.
The IVth/Max tower upgrade should provide an extra bonus, such as a 1 or 2 second decrease in reload time, or some extra effect like super range. For instance, the artillery and green circle laser weapon should get a reduction in reload time at the final upgrade. It only makes sense to reward the user for putting so much money into a tower.
As someone stated before, as the level progresses, the enemies should provide you with a bit more money. Not a lot more, just enough to not get so overwhelmed.
A good feature to add: If a player is trying to put down a tower over some walls (the ones you can buy), let the tower build over the walls, and sell the walls it's covering. To avoid accidents, this could require a double click.
Also, I don't know why, but there seems to be some tower missing. Not saying you forgot one, i just feel like there needs to be one more; Like a bonus tower that doesn't attack, but increases the range or damage of nearby towers, you know?