Really cool game.
You can exploit a retreating enemy to keep backtracking in the last wave, receiving emeralds each time. With rings of health or enough healing spells you can aleays exit the third level at full HP.
After running for a while lag spikes started ocurring, and eventually a popu saying something like "Something lost conection, please reload the page" appeared and the game froze.
As for design feedback, since you know can see the cards that aren't on the table and with that information you can find out which are, it would be nice if there was a list with all the enmies and items currently hidden so you don't have to make such annoying calculations yourself.
Also, I don't like having to waste upgrades to know how each weapon improves.