This is a great game. The towers are nicely balanced, and having to deal with flying units and resistances adds a lot of strategy to the game. The Star upgrade system makes the game more addicting and exciting, and the way barracks checkpoints add user interactivity to the game is genius. I also applaud the upgrade level constraint in the challenge stages; it was very much a good idea that kept them as challenges.
My one qualm with the game is that the second and third upgrades for the polymorph and death-ray abilities on the two mage towers are significantly underpowered. While an auto-kill every 20 seconds is certainly worth 350 gold, speeding it up by 4 seconds is not remotely worth 400 gold (200 per upgrade). Consider: a cast every 16 seconds is 3.75 casts per minute, which is only a 3/4 cast per minute improvement over the original spell (with a 20 second cooldown); not a 400 gold value.
In my opinion, you should add a bonus level for getting every level completed in gold time and one for getting every achievement. That way players get to feel rewarded for all the work they put into getting those things...
About half an hour into playing this, I noticed that the gameplay had 2 components:
1) run around in circles avoiding enemies, and
2) Aim a gun at them and left click, over and over...
That was the point when I quit.
The levels are similar enough that there is no sense of progress, and it gets boring after a point. Personally, I quit after lvl 10. Add an incentive to continue, and this will be a great game. Even as it is, 4/5
Quite Buggy. Also, not very user friendly (for example, it is impossible to see ability stats), and the attacks that paralyze you are simply annoying. It is difficult to browse characters and new characters are not highlighted in any way. Last but not least, the game gets highly repetitive near the end, and the game degenerates to rushing each level with a flying character.
2/5.