In-game firefights are a bit hard to see (projectiles don't contrast enough with the background and are too small). I'd like to be able to see how much gold is needed to build a unit/tower, rather than just "Not enough gold". As far as the units go, there shouldn't just be better units. There should be a variety of units you can combine. Find a way to make the weakest unit useful for a greater duration, for example.
Interesting game, although you can spin in a circle for the first 3 levels and survive easily. A few tweaks that would make it better: increased framerate (from what I've seen of making games in Flash, this could be painful) and smoother controls, which seems like it would just be the framerate (i.e., it looks somewhat jerky when turning). 5 out of 5.
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