[Zigûr]

I remember this bit but there's no explanation for it in the theatrical release of any of the films.

I have a feeling it might be explained in an extended edition (none of which I've seen for the "Hobbit" films) perhaps with some reference to the Grey Mountains or something.

There is (to the best of my knowledge) nothing in the real story from the books to suggest that Thranduil had ever encountered dragons in his life, although I suppose it's possible.