Fun game. Bit of puzzle logic and math, but not enough to be annoying. Great art, the only tiny critique I have is that When you drag a card, it's a bit distracting to have it all the way off to the side. In addition, while it's nice that there's a predictable pattern to the battles/rewards/upgrades, it gets repetetive very quickly. Perhaps if you added some sort of goal, or a way to ramp up the action?
Overall a pretty good game.