Thank you for your feedback! I think a map would really have been helpful here :D After beating the Asteroids you could go left, where you collect the coin and go into a cave. In the cave you can go up again through a little maze. Since I fear more people are struggling to end the game, here is a little spoiler: After collecting at least 5 coins you have to return to the starting area, but if you manage to find all coins, there will be an optional upgrade