Fun but didn't stay that way.
The easiest way to do this is just to make a list of all the problems.
1. As has been said, buying health costs too much and is ridiculous that the price rises.
2. Health, and Armor upgrades did nothing.
3. There was no damage or effect difference between the elemental arrows, making ice arrows the clear choice.
4. Hit detection problems from enemies attacking the player. (i.e. the player could be hit while not close enough to the enemy)
5. Picking up money from fallen enemies is not fun. An automatic gold collection system would be more appropriate.
6. My absolute biggest problem with this game that made me stop playing was the poorly designed summoners. These guys took far too many shots to kill, had seemingly no limit on the number of spawnable minions, and came in far too large of groups. The boss that spawns summoners was even worse because the massive summoning spam would keep the boss protected and would severely lag the game even at the lowest settings. At that point I quit.
Overall, I like the concept. The upgrades need changes to make them actually work and there should be some more options for bows, arrows, and skills. The achievements were very nice and I had no complaints there. I think it ould be nice to have another mode or to as well. And maybe a better story for the main campaign.