I love this! The colors, sounds, and sprites are great. I made it to level 19 on the first difficulty.
I played with the turrets in single player, and I wish there was a timer so I could better anticipate when a turret was going to be placed. I'd like the health pack pick up range to be a little smaller so I could save them more strategically. I could see that you had a category label on the upgrade cards, but coloring the whole card might make it easier to see.
I haven't tried the multiplayer, but I think that's a really cool idea.