I really love a good tower defense game, and this is a seriously awesome idea. I see so much potential in it.
I thought it was really good and creative; I had a lot of fun.
My only complaint is the lack of a menu... it starts right away before you have any idea what's going on, and there’s no reset button—not even at the end.
Also, I didn't notice the enemies getting stronger... I didn't feel any difference after 10 minutes. I lost around the 15-minute mark, though I did neglect the colors a bit, so I'm not sure if I could have played much longer. Plus, there are no rounds or a pause button to take a quick break.
Oh, and I didn't really notice—or pay attention to—the differences between the towers... I just dragged the colors into empty slots regardless of whether they were blue or red... though I did gather that gold is supposed to be a bit stronger.
But overall, it's a really cool game and, in my opinion, genuinely creative. I had fun.