It did take me a while to pick up that I could use Mouse 1, 2, or 3 to cast my cards instead of stretching to #1, #2, and #3 while moving with WASD. Once I understood I could use the mouse (despite the keypad icons above the cards) the game became very enjoyable.
Also, the minute I wished that there was another enemy type one of the ones that could shoot spawned. So nice work there.
The music and sound effects and feedback were spot on.
I think you should polish this one up. I had a ton of fun.