Now I know why smelting is a problem. After you get Iron and new Tools, you can only smelt Iron with Coal. The Problem: I can not smelt Copper anymore?
Game gets boring, but its a good idea, neat look.
Add some story, not only as a prologue. Add some changes in the fight...maybe some superpowers, anger, a sword...
Really well done game! I would give some tips: Add a storyline with new enemies with other looks and add an upgrade for the humans' turrets/station.
22798 Points on hard ;)