The Great Wall of China is considered to be one of the Seven Wonders of the Medieval World. Medieval is defined as the period in history between Antiquity and the Renaissance, often dated from 476 CE to 1453 CE. And while the bulk of the Great Wall was built during China’s Ming Dynasty (1368 CE - 1644 CE), the first pieces of it were put in place long before the Middle Ages, during the Spring and Autumn Period (770 BCE - 476 BCE) of the Zhou Dynasty.

During the Spring and Autumn Period, the small Chu State gradually grew by conquering its weaker neighbors. To maintain control of its boundaries and prevent intrusion by the Jin and Qi states, Chu State built a series of walls along its northern frontier around 656 BCE. When the Zhou Dynasty entered the Warring States Period (476 BCE - 221 BCE), Chu State integrated its boundary markers and extended the resulting solid wall.

Around the middle of the Spring and Autumn Period, Qi State began construction of its own wall, a project that lasted 300 years, ending in the middle of the Warring States Period. Qi State’s wall also was intended to prevent invasion by other states and outer tribes. It stretched across almost the whole of Shandong Province, from Changqing County in the west to Jiaonan County in the east, passing through eight cities to reach the Yellow Sea.

Battles between states occurred often throughout the Spring and Autumn Period and the Warring States Period, and the Huns constantly assaulted China. To protect themselves, the states of Qin, Han, Yan, Zhao and Wei eventually built and extended their own defensive walls. By the end of the Warring States Period, walls existed in the Yangtze River valley, the Yellow River valley and the Northern China areas.

When Emperor Qin Shihuang established the Qin Dynasty (221 BCE - 207 BCE), he unified China and ordered the separate walls to be joined and further extended to ensure peace for his people against the Huns. The project took about 10 years and involved backbreaking work by tens of thousands of people, including slaves, convicts and conscripted soldiers.

The Great Wall of Qin consisted of three sections. The eastern section, mostly built on the wall of Yan State, started in Huade County, Inner Mongolia, and ran through Hebei Province to Fuxin County, Liaoning Province. The middle section, with beacon towers, mostly built by the Qin Dynasty, rose from Xinghe County, Inner Mongolia, in the east and reached the desert. The western section went along the Yellow River, beginning in Min County, Gansu Province, and winding its way to Yuzhong.

Some successive dynasties made extensions to the Great Wall, but another major project was not undertaken until the Jin Dynasty (1115 CE - 1234 CE), when two important sections were added to prevent incursion from the Mongols. Although the wall continued to serve as an effective physical barricade, when the Chinese government weakened from within, the Mongols were able to breach it and take power, establishing the Yuan Dynasty (1271 CE - 1368 CE).

After the Mongols were overthrown, the Ming Dynasty (1368 CE - 1644 CE) was founded, and the new Emperor Zhu Yuanzhang worried that the Mongols would try to regain control. Therefore, he ordered reconstruction of the Great Wall to strengthen, improve and extend it, using advanced civil engineering and military architectural techniques.

During the Ming Dynasty, the gigantic project was never neglected and the Great Wall as we know it was finally completed.

One of the largest construction projects ever, the Great Wall extends 6,700 kilometers across northern China, from Shanhaiguan Pass near Bo Hai (Gulf of Bohai) in the east to Jiayuguan Pass (modern Gansu Province) in the west. It has an average height of 10 meters and an average width of five meters.

Construction of the various sections of the Great Wall depended on local resources for materials and on current technology for methods. However, a common factor in sections from every place and period is maximum use of natural terrain, such as steep mountains, river gorges and narrow passes, as part of the wall’s design. The resulting achievement is a testament to the ongoing ingenuity and tenacity of the Chinese people.

Generally, the foundation of the wall is made of compacted earth. Workers would build a simple wooden frame, fill it with dirt, and tamp the dirt into a compact layer about 10 centimeters thick. The process would be repeated layer upon layer. Sometimes reeds, sticks, gravel, stones or other debris were mixed with the dirt.

The Ming added exterior layers of kiln-fired bricks and stone slabs to the basic tamped-earth interior. Workers mixed lime and sticky rice to use as mortar between the bricks. The frequently visited Badaling section of the wall in Beijing was constructed in this way.

Weapons of the periods included spears and swords, lances and halberds, and bows and arrows. Different dynasties added features to the wall that not only would defend against enemies with such weapons but also would accommodate their own soldiers using them. Primary among these features were passes, watchtowers, signal towers and moats.

Situated at key positions, usually on trade routes, passes controlled entry to and exit from Chinese territory. Their designs varied from a simple double wall to a virtual fortress with a maze-like format. Gates on either side of the wall were closed by enormous double timber doors secured with huge iron bolts and locking rings. Within the pass, ladders and ramps provided access to the top of the wall for both men and horses. Ramparts atop the wall had outer parapets to provide cover for archers.

More sophisticated passes had two to three-story watchtowers above the gates, allowing defenders to monitor the approach of travelers or marauders. More vulnerable passes might have a semicircular or polygonal wall shielding the gate and even an external tower to serve as a post from which troops could be directed during hostilities.

Moats created as soil was excavated to fill the wall added another line of defense.

Taking full advantage of hill tops and other high points along the wall were signal towers, also called beacons or beacon towers. These enabled soldiers to warn other garrisons of enemy movements and to summon reinforcements in the event of an attack.

Coded smoke signals were sent during daylight hours, with different materials burned to create various colors and densities of smoke. At night, fires and lanterns were used. Other means of signaling included flags, clappers, drums and bells. The invention of gunpowder greatly enhanced the signaling system, because the firing of cannons made a sound that could carry over a long distance.

Construction of military fortifications on the wall peaked during the Ming Dynasty, when shelter towers or blockhouses were built, with large interiors for storing food and weapons and for housing soldiers. Some 1,200 of these structures existed between Beijing and the Shanhai Pass. They had small windows for lookouts and cannons for artillery. Rusting iron cannons may still be found along this section of the wall.

Because of its architectural and historical significance, the Great Wall of China was declared a UNESCO World Heritage Site in 1987. Much still needs to be done to preserve it, though.

In December 2002, after a survey of damage caused by nature and man to 101 sections of the wall in different provinces, the China Great Wall Academy reported that less than 30 per cent of this must-see attraction for visitors to China remains in good condition.