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. The ancient wall circling the city stretches for more than 1,700 meters, with a height of 12 meters. 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)