I like the card battle mechanic, but the styling on the cards is a bit confusing. I think a button to "inspect" a card that would explain what the symbols mean would go a long way without removing the unique art style of the cards and symbols.
I eventually hit a point where I couldn't play or fuse any cards even though I had a green o, I'm not sure if it was a bug or if I just didn't understand the mechanic. I love the hand drawn art as well, that must have taken a long time for all the cards and characters!