Your pricing is maybe the worst I've ever seen. To upgrade stars of just one artifact on one character from level 4 to level 5 would cost me around $60. FOR ONE ARTIFACT??! This isn't greedy; it's just stupid.
The fist boss level is ridiculous. I have the max upgrades I can have and just barely make a dent in the boss. I have all 20 lives left, only let the boss through and I lose. Needs a major fix.
One big problem...I don't understand how the weapons target the enemies. I've made it to level 15 on the last stage 10 times, only to watch my weapons completely skip attacking the enemies who are destroying my HQ. Instead, they're attacking emeies who have no chance of making it to my HQ. Frustrating.
I got further on the medium levels than I could get on easy. The amount of damage the towers show they do is not the amount of damage they actually do. That would have to be fixed before I ever played this game again.