User blog comment:IamJakuhoRaikoben/Chapter 375: Supernatural, Review./@comment-3424593-20140314111920/@comment-7418318-20140314134411

We still don't know who build it. It could have been Humans, Demon, Dragons, any other intelligent species that lives in Erath Land (like the Frogmen serving the council) or another race that went extinct between Face's construction and today.

Or it could even have originally been build in Edolas and was somehow transported to Earth Land.