Cool take on the tower defense genre!! The card mechanic adds variety and gives each run some unpredictability. My biggest suggestion would be to speed up the pacing a bit, or have a speed changing option like sims or somethng.
But i found the using cards on towers mechanic fun, i can imagine a late version of this game where you are flinging cards regaining mana drawing new cards or something.
With some extra polish and balancing, this could become a really fun game. Nice job!