Game isn't bad, but the upgrades lack some punch.
After all, once you get the mechanics at the start, you focus on making money then upgrade all, and you never use the low-level battle upgrades.
It would be neat if each area had a shop, for higher level things, so it makes sense to progress bit by bit instead of just.. maxing all up at the start.
Maybe also shortcuts, so if you die, you don't need to do ALL the areas from the start, area-specific upgrades that lower the number of enemies you need to progress. Maxing everything is too quick and after that there's no feel of progression.