flatland. A formal forming system came into existence during the reign of Yixiulie, the eleventh king, when he devised standard measurement units for allocating farmland and counting livestock. Six palaces were built in the region between the rule of the ninth and fifteenth kings. Around the 6th century AD, the Yarlung Valley became a society which smelted a variety of materials including iron, copper and silver to produce weapons and other objects. The 31st king of Yarlung, Namri Songtsen, was an expansionist and invaded neighbouring tribes, expanding the territory of Yarlung. The 32nd king (who was also the first emperor of Tibet), Songtsän Gampo, conquered Subi tribe, Yangtong tribe and many others and established the Tibetan Empire. Although transferred his capital city from Qiongjie to Lhasa, many of his descendant of the royal lineage still lived in the Lhoka region. He made Yungbulakang Palace his summer palace, which by this time was an important centre for Buddhism and the storage of Buddhist scriptures. In 641 AD, Gampo married Princess Wencheng of the Tang dynasty. During the Tibetan Empire, a great number of irrigation projects developed along the Yarlong River and the Yarlung Tsangpo River, and with the increase of yaks and horses here there was dramatic development in agriculture. However, continued conflict led to endless famine and eventually the collapse of the Tubo regime. In 1253, Möngke Khan, the emperor of the Yuan dynasty, invaded Tibet and united the squabbling tribes into an administrative region, controlled by the central government of the Yuan dynasty in Beijing. However, the dominant tribe in the Yarlung valley during this period was the Pazhu dynasty and in 1322, Qiangqujianqun became the leader of the Pazhu and replenished agricultural activity in the region, funding irrigation works, cultivating lands, reconstructing roads, restoring houses and developing husbandry. Qiangqujiangqun founded the Naidong dynasty which would last for 262 years, introduced the system of feudal serfdom and implemented a new government system with divisions known as Zong, established 13 of them in Nêdong (the capital), Gonggar and so forth. On May 23 of 1951, a Tibetan delegation signed an agreement with the central government of the People's Republic of China for the "peaceful liberation of Tibet". However, in 1959, the Lhoka was annexed to China by brute force. Many monasteries were destroyed during the invasion and the Yungbulakang Palace was severely damaged. Geography Lhoka region forms one-fifteenth of the