Aerial photo taken on April 18, 2019 shows the Huangyaguan Great Wall in the northern suburb of Tianjin, north China. The Huangyaguan Great Wall was built more than 14 centuries ago for border defense purpose. The structure winds its way for about 3,000 meters along the mountain areas of today's Jizhou District on the outskirts of Tianjin. (Xinhua/Li Ran)