I really enjoyed playing this game. I thought it was very well done and I greatly enjoyed working towards the upgrades.
However, I didn't realize that the skulls and arrows meant that that room would be a boss battle. I didn't realize until I picked up all possible upgrades and had no way of progressing.
Nevertheless, I really enjoyed this game, but I think it would have been great if after having searched through a zone, a map could have been created as each area was discovered.