This is very impressive for a game done in one day! The art is great. Some thoughts I had about the mechanics:
- Seems like the 3 cost lightning strike was the most efficient in terms of damage/cost. The 5 cost assassin's dagger didn't impress much.
- There's an exploit where when you're out of energy you can still draw as many cards as you like.
- Shields were really important!
- The energy crystals felt unpredictable and I didn't feel like the player could plan ahead with them. So it made the game feel a bit more luck based than it should.
If you're thinking of expanding this game I think a tutorial stage at the beginning would help a lot, since I had to do a lot of trial and error to understand what I could do.