Bogo; in addition, Silla people made overseas communities in China on the Shandong Peninsula and the mouth of the Yangtze River. Later Silla was a prosperous and wealthy country, and its metropolitan capital of Gyeongju was the fourth largest city in the world. Buddhism flourished during this time, and many Korean Buddhists gained great fame among Chinese Buddhists and contributed to Chinese Buddhism, including: Woncheuk, Wonhyo, Uisang, Musang, and Kim Gyo-gak, a Silla prince whose influence made Mount Jiuhua one of the Four Sacred Mountains of Chinese Buddhism. However, Later Silla weakened under internal strife and the revival of Baekje and Goguryeo, which led to the Later Three Kingdoms period in the late ninth century. Unified dynasties In 936, the Later Three Kingdoms were united by Wang Geon, a descendant of Goguryeo nobility, who established Goryeo as the successor state of Goguryeo. Balhae had fallen to the Khitan Empire in 926, and a decade later the last crown prince of Balhae fled south to Goryeo, where he was warmly welcomed and included into the ruling family by Wang Geon, thus unifying the two successor nations of Goguryeo. Like Silla, Goryeo was a highly cultural state, and invented the metal movable type printing press. After defeating the Khitan Empire, which was the most powerful empire of its time, in the Goryeo–Khitan War, Goryeo experienced a golden age that lasted a century, during which the Tripitaka Koreana was completed and there were great developments in printing and publishing, promoting learning and dispersing knowledge on philosophy, literature, religion, and science; by 1100, there were 12 universities that produced famous scholars and scientists. However, the Mongol invasions in the 13th century greatly weakened the kingdom. Goryeo was never conquered by the Mongols, but exhausted after three decades of fighting, the Korean court sent its crown prince to the Yuan capital to swear allegiance to Kublai Khan, who accepted, and married one of his daughters to the Korean crown prince. Henceforth, Goryeo continued to rule Korea, though as a tributary ally to the Mongols for the next 86 years. During this period, the two nations became intertwined as all subsequent Korean kings married Mongol princesses, and the last empress of the Yuan dynasty was a Korean princess. In the mid-14th century, Goryeo drove out the Mongols to regain its northern territories, briefly conquered Liaoyang, and defeated invasions by the Red Turbans. However, in 1392, General