Excellent game... but
There are a few things though that must be addressed.
1. The lenght of each level - It is so long, it takes around an average of 10 mins to finish a lvl.
2. Birds - The only thing that's fighting you. They can get very boring quickly due to the lvl lenght.
3. Spells - quite good, I like the spell buying concept. But, since you can only get a few gold per kill... it can take a while to reach the end. I don't even know if you can purchase all the upgrades by lvl 13.
Other than that, great job.