A fun idea, combining deck building with tower defense. Thanks for the tip about the Tank turret, it really saved me and let me get to the win screen!
I had one run where my only cards on the second wave were Remove Turret which didn't work out so well for me.
I think it would be helpful if there was a brief pause after enemies spawn and before they start converging on the player, so that there's time to look at both sides and decide which one needs more defenses. I (think?) as it is now there's always an equivalent force on each side so all the player really needs to consider is which direction has the weakest defense, but in a longer, more complex game I found I could only scroll over to look at one side before I needed to place turrets. Alternately or in addition, being able to 'snap' the camera to the furthest left and right areas would help, the same way that Space snaps it back to the center.