Around the 6th century AD, the Yarlung Valley became a slavery society, smelting iron, copper and silver to produce weapons and other objects. The 31st king of Yarlung, Langrilunzan, was an expansionist and invaded neighbouring tribes, expanding the territory of Yarlung. The 32nd king, Songtsän Gampo conquered Subi tribe, Yangtong tribe and many others and established the Tubo Regime, or 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.