For those who are wondering it's kind of a Castlevania/Metroid style game. Except you keep returning to the town once it gets harder for you to dig down any deeper (due to needing more light, better weapon, etc) and upgrading your stuff while in town so you can go back down and go even further. It's addicting.
Any time you find treasure you can cash it in for upgrades like more health, stronger pickaxe, don't lose as much treasure when you die, increase pouch for holding treasure, etc. And they're all so equally important it's hard to choose!