With its iron frame, the flax mill is considered as the direct forerunner to the skyscraper and project officer Fay Bailey said it was of "international significance".
The mill was built around an iron frame in a technique adapted in the rebuilding of Chicago in the late 19th Century and for skyscrapers such as New York's Empire State Building.
Italian craftsmen learned how to precisely mill these reading stones, and by 1286, they were making lenses that could be held in a frame in front of a person's eyes.