The upgrade system is very unintuitive. It's exponential, yes, but in such a way where the first... dozen or so upgrades are completely overshadowed by one or two later ones. Not in a good way - in the "those first 20 upgrades amount to an extra 2% chance of dropping a coin" way. Enemy upgrades are also exponential while only giving linear coin upgrades. And your damage is linear (up to a point, where "attacking" damage gets up to a x2 multiplier) If you don't notice the maths, you will very quickly over-upgrade enemies into very bad spots, and won't know how to outscale them. I'd say either make the upgrades linear, or more forgiving. Allow to re-spec upgrades, for example. I also never found a way to sell stuff, even though I had thousands of bones in my inventory at the end. It's... fine, it's a fine game, but it needs some work.