Children play inside the Citadel in Derbent, Russia, on June 28, 2019. The Citadel, Ancient City and Fortress Buildings of Derbent, is located in Russia's Dagestan region, on the western coast of the Caspian Sea. The fortification was built in stone and consisted of two parallel walls that formed a barrier from the seashore up to the mountain. It was inscribed on the World Heritage List in 2003. (Xinhua/Bai Xueqi)