I really love the art style and the upgrade system! I wouldn't know how to fix this but my main nitpick is that the order of upgrades feels pretty set in stone i.e. you generally want to upgrade your mining speed first, then work your way down to upgrading the turrets which seem to be able to hold off the waves pretty well so long as you keep them energized. I wish there could have been more balance in the priority of choices but that might be asking too much for a 4-day jam. Really cool stuff!