I commend how far this got in such a short jam period, however, I think it may have been a bit too much to bite off for a week long jam. I'd love to see a more finished product. I echo others thoughts regarding the need for a tutorial. It isn't clear how defense works at all. For example my 90 attack card attacked an enemy card with 45 def and 70hp. this resulted in 16 damage (54 hp remaining). This must be explained to the player somehow because it does not make sense intuitively - id expect 45 damage to be "defended"/ ignored.
Also not a big fan of the random amount of mana regained per turn, RNG in deck builders should come from which cards show up, not core mechanics, IMO, unless some special high risk high reward ability ought to have a coin flip or dice roll for outcome. I hope this doesn't read as too harsh, I'm only writing so much because I did enjoy the concept, im impressed with what there is so far, and I would love to see it more fully fleshed out.