First off, this is a very fun game. It seems a little too easy at the beginning, and then when you get to around 130 waves, it becomes incredibly difficult. It might be better to change the difficulty levels a little more evenly.
I like the game overall. It seems that the computer opponent gets better cards than the player most of the time. All things aside this is a great game. Levels would make it even better. Particularly if it got harder as you leveled up. Maybe giving you new cards as you level up.
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