By the early Southern Sung, after years of training, the Sung army made major advances in terms of military organization and tactics. In organization, the Sung army established divisions purely on the basis of weaponry, such as archers using crossbows. As for military tactics, the army developed the system of "accompanying archers." Archers in the Sung military rose to prominent position with their great achievements in battle. In fact, their use of the crossbow made major contributions to the preservation of the Sung state in the south after the fall of the north to the Chin in 1126. The figure of Wu Lin also stands out for his creation of a kind of layered formation, which he often used with success in field skirmishes. Layered formations involved using to a great extent the crossbow in field formations and skirmishes. It proved to be a major offensive weapon for Sung troops, and it also served to thwart the forward advance of Chin forces in field skirmishes. The crossbow in the Southern Sung still mostly was represented by the quick-set bow. Its production as well as power reached unprecedented heights at the time. Generally, the Southern Sung reflects the peak in the development of the Chinese crossbow. Unfortunately, by the latter Southern Sung, generals only achieved effective use of the crossbow in the defense of cities and naval warfare. They gradually neglected the role that it had played in dealing with the enemy in field skirmishes. Thus, in fighting between Sung and Mongol armies, Sung generals often suffered defeat at the hands of Mongol calvary in open battles. The inability of Sung generals to come up with effective ways of dealing with the Mongol calvary was one of the major reasons why the Sung was ultimately overcome by the Mongols, who then set up the Yiian dynasty in China.