This is a really neat concept! I think this is a creative idea that would be interesting to expand upon.
For some constructive criticism, I think first and foremost this could use some more clear instructions for the player. I was unsure how the cards behaved, where I was supposed to place them, and how the projectiles would behave. Having some built-in instructions and maybe some sort of legend for how the different card types behave would be a great improvement.
I also had some visual issues playing this in browser. The display was very small and it was a bit difficult for me to see the cards.
All in all I think this is a fun idea at it's core and is a pretty cool prototype for a 3 day jam. Well done!