The rulers in the Koryo Dynasty adopted Chinese ways while still trying to maintain Korean ways. Only the nobles were allowed to take the civil service test and government positions were inherited. In result of this, there was a major divide between nobility and anyone else. Culture thrived in this period. Artisans made blue glazed ceramics called celadon. The Koreans improved on woodblock printing with metal movable type. In the 1200s, the Mongols invaded Korea. They enslaved many and forced rulers to pay large tributes. They also took many to China to serve in the Yuan military. When the Yuan was weakened, they rebelled and gained their freedom.