After equipping the copper armor, an enemy with a bow will appear.

The copper armor deflects arrows.
Defeating the archer spawns iron ore, which you can mine to obtain iron.
Smelt the iron in the furnace to craft a gear.
Combine the gear and a bow to make a crossbow.
The crossbow is a rapid-fire weapon—hold E to shoot continuously.

Once you've crafted the crossbow, a mounted enemy will appear.
Be careful—the horse-riding enemy moves fast!
Defeating them drops gunpowder.
Combine gunpowder with the crossbow to craft a musket.

After crafting the musket, four enemies in copper armor will spawn.
The musket can pierce through their armor.
Once you defeat them, a wrench will drop from the sky.
Combine the wrench and a furnace to build a steam engine.
Insert the musket into the steam engine to craft a rifle.

After crafting the rifle, a swarm of enemies will appear.
Defeat them all!

Once cleared, a circuit board drops from the sky.
Combine it with the steam engine to craft a computer.
Insert the rifle into the computer to build a missile.

To use the missile, hold E to begin locking on, then release E to fire once locked.

After crafting the missile, a spaceship appears.
Defeat it to obtain an energy core.
Combine the energy core and the computer to build a time machine.