In 221 B.C., King Qin Shihuang defeated the other States, unified the whole China, and established the Qin Dynasty, also knowns as the First Empire, the first centralized empire in China. A long-time division by feudal lords ended.
The territory of the Qin Dynasty enlarged greatly, with its north border extending to present east Liaoning Province, Yinshan Mountains and the Great Bend of the Yellow River, the east border to the sea, the west to now Qinhai Province and the Gansu Plateau, and South to now Guangdong and Guangxi Province. The Qin Dynasty abolished other characters, laws and metrologies. To eradicate the influence of division left by the warring states, it replaced the enfeoffment with prefectures and counties and torn down the walls and embankments that blocked connections.
Hun, one old minority nationality from the north of China, began to thrive in the late Warring States War Period. When the Yan and Zhao States were falling off, Hun invaded them and bit by bit occupied large patches of land in the Great Bend area of the Yellow River. Hun migrated with seasons and based their agriculture mainly on collecting plants and hunting. This tradition made their troopers fast, depending on which the Huns looted labors and wealth on Qin's border. Hun damaged farming work and setting the north border of the Qin Dynasty unrest.
To solve the border trouble, Qin Shihuang appointed General Meng Tian for garrisoning troops on the north. In 215 B.C., Meng Tian led an army numbered 3,000,000 and assaulted Hun successfully. The battle returned present the south area of the Great Bend of the Yellow River of Southwest of Mongolia Province. Moreover, he swept crossed the Yellow River and took up present Linhe County, the Yinshan Mountains and the area north to the Wujiahe River, and set in these places 44 counties. The war forced Hun out of the Wall of Zhao and eliminated its threat over the Qin Dynasty.
Qin Shihuang later found that the walls of the Yan, Zhao and ex-Qin States were disconnected from each other and could hardly stop enemies from breaking in again. So in the year 215 B.C., he ordered to link up these three walls. The weather-beaten parts were also reconstructed and new parts were added in some places. The labors for this construction numbered 2,000,000, made up of the army under the command of Meng Tian, confiscated labors, captives of war and the guilty people against laws of that time. The whole construction lasted for 10 years.
The finished wall extended further at the north end as the territory of the Qin Dynasty in the north had expanded. The wall started at Lintao, i.e. Minxian County now, went eastward to now Guyuan of Ningxia Hui Autonomous Region, met the Wall of the Zhao State that ran eastward from Gaoque of the Great Bend of the Yellow River, went on and crossed the Yellow River, entered now north Shanxi and Hebei Province, ran the rest part of the Wall of the Zhao State and headed on along the north branch of Yanshan Mountain, passed by way of the five counties of Shangu, Yuyang, Right Beiping, West Liao and East Liao, and finally arrived at Jieshishan Mountain near the Datongjiang River of Pyongyang, Korea. This wall was longer than and lay slightly north to the Great Wall we see today, which was built by the Ming Dynasty about 1,500 years after.
Since the Liberation of China, the Chinese government has launched several investigations on the Great Wall of the Qin Dynasty along the route that passed by Minxian County of Gansu Province, Langyashan Couty, Yanshan Mountain, Chifeng of Hebei Province till entering into Jilin Province. The relics of the wall can still be seen scattered along the route. Some parts relatively well preserved are about five or six meters high, made of blocks of mud or stone of the local resources.