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In 1268 A.D., Kublai Khan, emperor of Chinese Yuan Dynasty, sent two of his generals — Liu Zheng and A Shu — led their armies to attack Xiangyang, which marked the launch of the war against Song Dynasty. Mongolia decided to seize Xiangyang at all costs, increasing armies to over 100,000 people in just one year. Xiangyang was built with the Han River as its natural moat and with a brilliant structure outside its each gate: a barbican. Sandbox is a security mechanism that provides an isolated environment for suspecisious programs.