I really liked this game. Deck builders are some of my favorite games and I think having unlimited upgrades per card and having the player choose between upgrading cards or deck management is a great strategic feature. One thing I would like to see is some evidence other than text that says a card was upgraded. Like, changing the border color of the card or adding stars/gold bars to the side, etc.
The level design was A+, I really liked how I needed to really explore to find the switches, the next area, etc but the levels didn't overstay their welcome.
I LOVED loved loved loved LOVED the transition into the underground area. It was so sick! Really above and beyond for a jam game.
I haven't completed the game yet, my browser keeps "reloading the page because it's using too much energy" and the game restarts when that happens but I'll switch to my desktop PC later and give it a try there.