This is a cool concept with a nice, simple gameplay loop. The art and general presentation is also nice. It didn't take very long to understand how everything worked. Graphically speaking, it's very polished, which I liked a lot.
I do think the game has some balancing issues. The early-game often ends up being very luck-reliant, since there's a nice chunk of cards that will guaranteed end your run if you fight them. There might generally be some benefit to having a weighed random of sorts, like having it so that higher-damage enemies are more likely to spawn on the right-side cards, or having face cards/aces being added to the deck as you progress through floors, such that there's a little less all-or-nothing right at the start.
Moreso, there's a lot of snowballing with player level. Since leveling up heals you and increases your damage, and that in turn makes it easier to fight more enemies and get more XP, it results in a higher-leveled player gaining levels much more easily. If it's too hard to fight all the enemies, it'll only get harder. Meanwhile, if it's easy to blow through enemies it'll keep getting easier. My two successful runs had me consistently 1-hitting every enemy and 2-hitting every boss starting from floor 3. I'm not sure what would best help with this, though one of my suggestions would be to forego XP on diamond enemies, since the money still seems to be a good reward there; If it's meant to be a negative effect, the extra XP is definitely a more positive outcome.
Good stuff overall, very cool idea.