Easily won at the second run. Upgrading gold mine starts some kind of chain reaction, because it's exponential (multiplied each turn). Maths are needed here to balance things.
Maybe instead of a +1 upgrade for gold mines, just a chance to get a +1 on a gold mine to start smaller for example?
I feel there is a base for something cool. Maybe add more choices to the map exploration? For now it's just choosing between gold, attack or health. And the position in space has no use for now, it almost just like a tree you explore.
Finally I really don't understand why moving around the camera is on the inconvenient middle mouse button while the right click is freely available.