I really enjoy this type of games!
I think there is some small balancing to be done to the cards. Like the card that costs 2 mana to get mana from all controlled sources. That card is free mana. Feels almost like Pot of Greed. I think it would be more interesting if it had some condition to activate or maybe some gold cost?
It would also be nice to see the possible places to use a card before buying it when it's hovered with the mouse.
Other than that I played a good couple hours of this game. I enjoyed thinking each turn about what to do. Very nice and interesting game!