Amazing! Very interesting and creative idea, loved it and would like to see more in the future!
The only complains I have, is energy regeneration. I feel like it was too harsh, i was hardly being able to mine coal and later was mainly running from tons of enemies. Also would like to see some kind of safe space from enemies, to just calm down and decide on upgrades because it was really hard to move goblins and fight at the same time.
Overall it is very impressive game having in mind such a short timeframe!