Aerial photo taken on May 28, 2019 shows a view of the Yongtai ancient city in Jingtai County of Baiyin, northwest China's Gansu Province. Construction of the Yongtai ancient city began in the early 17th century and the complex used to be a military fortress. Girdled by 12-meter wall structures totaling 1.7 kilometers in length, the Yongtai ancient city looks like a turtle when seen from above and is thus dubbed "the turtle city". In 2006, it was listed as a state-level key cultural relics protection unit. (Xinhua/Ma Ning)