Wow, what an excellent jam game! Very cool idea and nicely executed :)
You mention that you'll keep working on it after the jam, so I've got some suggestions that might help (though I fully understand that it's a jam game and you've probably got 1000 things that you wanted to put in the game)
- Speedup would be great, especially early on.
- So would music/sfx volume control/muting, and
- range indicators on the towers.
- For design itself, something I noticed is that because the towers retain the same power over the course of the game, the stakes can get really low if you get some good hands early on. I have some ideas that would help with that.
- Adding gold to the monster drops, and allowing the player to:
- Wager money after seeing their initial hand, to power up the turret. Amount of gold could directly correlate to multipliers on that turret's power. So players could save up gold for one big tower or spread it out among all their towers.
- Make all of the different towers more similar in power, but allow the player to wager coins to play a different card game (blackjack?) to upgrade them later.
- Letting the player spend a certain amount to redraw cards in any of their hands.
- Adding the ability to "mulligan" their first hand, as I found it was often just worth restarting the game if I didn't get a good hand to start with.
- Adding gold to the monster drops, and allowing the player to:
Sorry for the unsolicited feedback, I had a lot of fun with the game and I'm excited to see where you take it :)