Aerial photo taken on Dec. 15, 2020 shows a view of the Huifengzhai ancient stronghold on top of a loess ridge in the Yanchuan Yellow River Shequ National Geological Park in Yanchuan County, northwest China's Shaanxi Province. The stronghold built during the 16th century is one of the old defensive structures found in northern part of Shaanxi, now a tourist attraction. (Photos: Xinhua/Tao Ming)

Aerial photo taken on Dec. 15, 2020 shows a view of the Huifengzhai ancient stronghold on top of a loess ridge in the Yanchuan Yellow River Shequ National Geological Park in Yanchuan County, northwest China's Shaanxi Province.

Aerial photo taken on Dec. 15, 2020 shows a view of the Huifengzhai ancient stronghold on top of a loess ridge in the Yanchuan Yellow River Shequ National Geological Park in Yanchuan County, northwest China's Shaanxi Province.

Aerial photo taken on Dec. 15, 2020 shows a view of the Huifengzhai ancient stronghold on top of a loess ridge in the Yanchuan Yellow River Shequ National Geological Park in Yanchuan County, northwest China's Shaanxi Province.