I liked the game.
I have some suggestions if you don't mind...
Time speed should have less steps or an easier way of setting it should be implemented. You know it is too slow at first so you increase speed by 3 mouse clicks I guess, but when the enemies get stronger, you have to decide between slowing down the game and deploying a new tower.
Tower colors tell it all, that is nice. But I feel like upgrades cost same for all. The middle one (which can kill both blue and orange enemies) should have a higher upgrade cost.
BTW, I couldn't see how much an upgrade costs.
I know it's not finished, but adding an option to sell the towers would be nice, of course [Game HAS a selling option, but it is not obvious]
And, consider a way of showing how many upgrades a tower has (maximum).
When the special type of enemy is about to be spawned (invisible ones) warn the user so he can do something about it.
Of course, full screen or a bigger screen would be cool
Well, I translated the page into my language using google translate and I saw that you have shortcuts for everything
So, scratch the time speed setting thing.