Fig. 3: Study area.

a Geographic location of China; b Geographic location of Shaanxi Province; c Geographic location of 44 castles along the Ming Great Wall; d GDP of Yulin city; e Urbanization rate of Yulin city.

a Geographic location of China; b Geographic location of Shaanxi Province; c Geographic location of 44 castles along the Ming Great Wall; d GDP of Yulin city; e Urbanization rate of Yulin city.