After usurping the throne of the Later Zhou dynasty, Emperor Taizu of Song ( 960–976) spent sixteen years conquering the rest of China, reuniting much of the territory that had once belonged to the Han and Tang empires and ending the upheaval of the Five Dynasties and Ten Kingdoms period. In Kaifeng, he established a strong central government over the empire. The establishment of this capital marked the start of the Northern Song period. He ensured administrative stability by promoting the civil service examination system of drafting state bureaucrats by skill and merit (instead of aristocratic or military position) and promoted projects that ensured efficiency in communication throughout the empire. In one such project, cartographers created detailed maps of each province and city that were then collected in a large atlas. Emperor Taizu also promoted groundbreaking scientific and technological innovations by supporting works like the astronomical clock tower designed and built by the engineer Zhang Sixun.