Zombie Smash update

[mc games]

Finally trying to finish this game and get it to the App Store. I have it now as a universal app for iPhones and iPads. Just uploaded the most recent version onto the Hub. I've had a couple of friends play the game and they both thought it was too easy to advance from one level to the next. Would like to get other's opinions. Should I increase the score needed to unlock a subsequent level,or is it challenging enough as is?
Thanks for your input.