I love the concept. Music is fun but maybe having more than one soundtrack would break up the monotony. I wish there was a way to earn more coins because unless you spend time collecting coins, and not killing slime before the 7th level there is no way to save up enough coins to buy upgrades. I found the levels before the 7th fairly easy however the 7th level is very hard unless you have a gasmask. Maybe when going back to the main menu you could replay other levels to get a coins or something.
P.S. I couldn't complete level 7 because I didn't have enough coins to get the mask.