Need on writing a fairytale about the middle ages.

{I hope this helps if it doesn't let me know and I can try to give more help.}

I would look up old Grimms Brothers fairytales or Hans Christian Anderson fairy tales that haven't been really touched up on, put it in a medieval setting, and put your own twist on it. Not necessarily the same story, but let the already created story give you an idea of what you want to write. (That way you're clear from plagiarizing.)