Still playing it but wanted to give my thoughts after a few levels:
-The level pop-up effect is so cool. I've been playing through Divinity OS 2 again recently and it made me think of that - such a nice polished touch to have in your game that makes the encounters sudden and unforeseen. Love that.
--And polish, on a more general note, is top notch for the most part. Super creative use of assets. And the right amount of game juice. The sounds are chunky.
--The overall concept is super fun, too. A crawler/arena shooter where you go up the deck of cards? Awesome. If you were using your own art you could see how this easily lends itself to themed levels and the like, so overall that's super solid.
--One critique I have is that I'd make pickups automatic. It's so fast paced that I don't think you need to press E to pickup health and ammo. I think it'd keep the flow better if it was triggered on collision.
--I also think a bit more indication when you're getting hit would be nice - a screen flash our sound effect.
Overall, I really like it! Gonna get back to playing it now :)