This is the only explanation I've found so far on how to do this. I don't get it, could you please elaborate? What do you mean with "light three stones with images"? How do you burn chestnuts in fires, where do I find the fires and the chestnuts? What do you mean with "the castle itself"?
There is a passage in the walkthrough about this, it tells you to light the fires in front of the large stones with the 'images' (i.e., markings). It says "Start by interacting with the giant stone gate in the Forbidden Woods, this will active the fires by the runestones.". I've been unable to find this "giant stone gate" tho....