I really like the countering mechanic, it works really well and is very useful in combat. I also have a suggestion on the leveling up. The fireballs feel very powerful from the start, the sound and size just feels like there's a lot of power behind each hit. I wonder if it could improve some feel of the game to have the fireball spell start out looking weaker and slowly increase in size and power with each level. Just a thought.
It's a very fun game and I like all the art and mechanics!