Oh, fun to see a little card gaming here. This one could really use a tutorial, or at least some instructions. I wasn't sure at first if the number at the bottom of the card was its cost or the number of that card I had in hand. I had no idea there was a limit to cards played until I saw the upgrade screen. And a heads up that you aren't supposed to even beat any encounter until a few upgrades would be nice. The ability to undo cards played could also be cut, as it gets very confusing when you undo cards that drew other cards that you also played.
But once I understood what was going on, I liked that each of the encounters brought their own deck and strategy needed to beat them. Different upgrades were better for each. That's good design, and this concept could be expanded upon.