Overall, pretty good
My biggest complaint is that we can't see how a weapon tree will develop. I.e. I have no idea, when buying a bow, if it will unlock double arrows or bouncing arrows or flying sheep arrows. The same holds true for spells and abilities. Knowing what we're buying our way into is essential for planning how we're going to build our defense and tends to lessen frustration greatly. Also, it would help greatly were we to have some manner of explanation about exactly what the effects we buy actually DO. For example, what does piercing actually APPLY to? Just bows? Or to guns as well? And what about throwing weapons? Can I have a "piercing rock" for example? And what about bouncing? And how powerful is a poison effect, when compared to a fire effect? These are things I would very much like to know.
Still, the game is very fun overall. The bow was very good, especially when I upgraded it to the five arrow split. That was just disgusting. Though, in later levels, I seemed to hit a plateau and found myself frantically upgrading to a decent gun while fighting off waves of increasingly powerful robots. I'm still not quite sure if I should have been going for a stronger gun, or just dumping points into power... Some explanation of what each upgrade does would, of course, lessen this confusion greatly.