This is a good concept, but it feels like there's a lot that could have made this better. The gameplay as a whole reminds me a lot of the turret minigame in Wpnfire, which isn't necessarily a bad thing, but turns out to be in the end.
The two biggest issues with the gameplay are the small size of the screen and the panning camera. Much of my time was spent staring at the borders of the screen so that I did not accidentally click outside of them. It would have been nice if you could have just shown a zoomed out view of the screen so that my cursor never had to pan so close to the edges. On the topic of clicking, my Flash player actually crashed the second time I accidentally clicked the Stickpage logo. Was there NO WAY for you to simply remove it? Crazy Jay didn't even help make this game. The logo is hardly necessary.
Next, I appreciate the colored enemies, but because of how simple and easy switching colors was, I found the mechanic to be somewhat of a waste. I suppose if I had gotten far enough into the game for there to be four or five colors of enemies I would have appreciated it a lot more, but I simply quit at a certain point because of the above issues. Without that damned camera, this game would have been entirely different.
Last, I'll go into the shop, which I only visited once, since I didn't feel like restarting the game when Flash crashed. My first criticism here is just how damned long it took to get there in the first place. There are far too many waves between levels, and it makes the game pretty mundane. Second, the upgrades are frustratingly expensive. Personally I might have made the upgrades weaker and cheaper so that players could fine-tune their decisions a little bit more.
Overall I did enjoy the game. The graphics and music are also great, and stick figures once again save the day when it comes to eliminating lag. Just that damned camera...