What a clever game, great job! I think I've noticed a glitch though with the silver keys. The amount you collected is still there even after dying, allowing you to stack them up and complete the level without actually getting the silver keys in one go properly. Sorry I don't remember the name of the level, but here's a screenshot:
They still seem to persist going to the next level:
Exiting and reopening the game seems to reset the silver key amount. I don't know if there are plans to expand on this game but I thought I'd let you guys know about this just in case! Once again, nice clever game! :D