The game concept is pretty cool. However it felt pretty boring to me at current stage. I think mostly because of the lack of challenge. I have no reason to fight stronger monsters even if they drop more gold as it's pretty easy to obtain very strong cards at the beginning. You could unlock stronger cards with stronger monsters or you could introduce player level and experience. Every enemy can give you XP only to the certain player level.
I also don't like that shield resets on enemy death AND deck shuffle. I think you should keep one of these OR always shuffle deck after enemy death.