battle plans would be drawn up to specifically give the castle at zhangjiakou , in china 's northern hebei province , a wide berth .
now archaeologists have begun a daunting project to restore the once-mighty site to something approaching it previous glory .
among the works being carried out at wanquan castle is the restoration of the fortress ' dilapidated outer wall that repelled every attack which invading mongolian armies could throw at it .