What were the two important wall building dynasties of China?

The two important wall-building dynasties of China were the Qin Dynasty (221-206 BCE) and the Ming Dynasty (1368-1644 CE). The Qin Dynasty initiated the construction of the Great Wall to defend against northern invasions, linking and expanding existing walls. The Ming Dynasty significantly enhanced and rebuilt the wall using stronger materials like brick and stone, creating the structure recognized today. Both dynasties played crucial roles in fortifying China's northern borders.