Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Wang Yiliang)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Zhang Chenlin)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Ju Huanzong)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Zhang Chenlin)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Ju Huanzong)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Wang Jianhua)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Wang Jianhua)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Meng Chenguang)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Wang Jianhua)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Wang Jianhua)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Ju Huanzong)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Ju Huanzong)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Ju Huanzong)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Wang Jianhua)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Hou Dongtao)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Hou Dongtao)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Hou Dongtao)

Aerial photo taken on March 18, 2019 shows Jiuyanlou Great Wall in Sihai Town, Yanqing District of Beijing, capital of China. Jiuyanlou Watchtower, allegedly the largest of its kind on the Great Wall built during the Ming Dynasty (1368-1644), gets its name for the nine windows on each wall. (Xinhua/Wang Yiliang)