Six mountains in this world are hollow, and they are the only ones that will ever be: everywhere else is a heightmap and stays that way. Inside, you dig with a pickaxe — real digging, cut where you swing, and the hole you make is there for everyone, permanently.
The deeper you go the better it gets. Stone near the surface, iron below it, gold and emerald deeper still, and bedrock at the bottom that nothing will break.
Take iron to an anvil and forge a better pickaxe, which bites a bigger hole and works faster. Gold smelts into coins. The loop is chop, mine, forge, mine faster, build.
The anvil makes weapons too: swords, clubs, axes and daggers, in stone or iron. The material sets how hard a weapon hits — everyone starts with a wooden sword; stone bites harder, iron hardest. The kind sets how it fights: a dagger is quick and light, a sword is the all-rounder, a club is slow but knocks things off their feet, and an axe lands the heaviest single blow. Whatever you hold, let it recover between swings (the charge meter shows it): a hurried blow lands weakly.