Decent first version with large potential
It's become clear that defense games are very popular right now, however not many are worth playing. Yours is in fact, decently interesting because of the difficulty. Although I do have some criticism and concerns.
1) There are many bugs such as i) Pressing 'H' does not bring up a help screen like noted during gameplay, ii) If an enemy is located on the bottom most track it is impossible to attack them if they are right next to the fence without using a magic strike.
2) The experience gained during a battle stays even if the battle is lost and one chooses to retry. However if one gains a level during a lost match this level does not count and experience gained disappears. Either make the level count or restore the exp. number back to what it was before dying when a match is retried.
3)Take less emphasis off of the critical strike. During gameplay it's fully required to get every headshot dead on or else it takes much too long to kill an enemy and one will eventually lose. I suggest you make it so that you can kill an enemy with a normal strike, but a critical strike will speed that process. Otherwise make different difficulty levels.
4)It's much too short, as mentioned before. There are new items unlocked right as you finish the game. Like most players I want to enjoy everything I have unlocked and it is impossible to make enough money to buy everything with the limited amount of levels.
Good luck on the next version, can't wait to try it.