Thats actually impressive... most play testers had a hard time even finishing the game, let alone in under 15 minutes.
One of the things that helped was your background design. I realised that if I stopped in the middle of the darker squares I could "easily" squeeze in between spikes and not die. If one day you want to make a hell mode, make your background uniform XD Else, it was very good design on your part to help the player with such a simple thing (planned or not) :)