The Cave is full of ores that respawn in a specific time, which drop the minerals when mined. These ores will help you in your journey to defeating the boss in the level. 
With the minerals you can go back to the lobby to trade or craft with the people on the village.
Once you have a key you’re able to open the boss door. Each key is a try, which if you fail you’ll need to get a new one.
The boss has three stages, which each stage he gets stronger and changes his attacks. Overtime the floor will change from dirt to lava and to avoid it you’ll need to listen for the noteblock noise.
After the boss fight is the end of the demo. More levels and boss fights will come soon.