The Great Wall has an incomparable symbolic significance in the history of China. Its purpose was to protect China from outside aggression, but also to preserve its culture from the customs of foreign barbarians. Because its construction implied suffering, it is one of the essential references in Chinese literature, being found in works like the "Soldier's Ballad" of Tch'en Lin (c. 200 A.D.) or the poems of Tu Fu (712-770) and the popular novels of the Ming period.