When Neil Armstrong, the first American astronaut to land on the moon, looked back at the huge, glistening earth, he could identify only two manmade works: the dikes of Holland and the Great Wall of China.

The Great Wall is the largest defense work of ancient China and one of the wonders of the world's architectural history.

Construction of the Great Wall lasted for more than 2,000 years, from the Spring and Autumn (770-476B.C.) and Warring States (475-221B.C.) periods to the end of the Ming Dynasty (1368-1644). Stretching from east to west in northern China, the Great Wall rises and falls with the contours of the terrain, climbing over rolling mountains and passing through grasslands and deserts.

The grandeur of the Great Wall and the difficulties and long years of labor in building it are rare not only in the history of China but also in the history of the world. The Great Wall was listed as one of the world's seven wonders a few hundred years ago, alongside the Coliseum of Rome, the Leaning Tower of Pisa, and the Sophia Mosque in Istanbul.

During the Spring and Autumn Period, seven ducal states appeared alongside the Huanghe (Yellow) River. Chu State was the first to erect walls to ward off incursions and was followed by other ducal states. At that time, Yan, Zhao, and Qin states were often harassed by the powerful northern nomadic tribes, and they built walls and stationed troops on garrisons along their northern borders. Construction of the series of Great Walls thus began.

In 221B.C., Qin Shi Huang conquered the other six ducal states, unified China, became the first emperor of the Qin Dynasty (221-207B.C.) and built the first centralized feudal empire. To consolidate the territory and protect the country from the harassment of the northern nomadic tribes, he ordered the construction of the 5,000-kilometer Great Wall, starting from Liaodong Bay in the east and ending in Lintao of Gansu Province in the west.

There were Great Walls other than the one built by the Qin Dynasty. More than 20 ducal states and dynasties before and after the Qin Dynasty built walls in different areas. The Great Walls erected by the Han Dynasty (206B.C.-220A.D.) and the Ming Dynasty were each more than 5,000 kilometers in length.

The Great Wall of the Han Dynasty was the longest: 10,000 kilometers. It was built on top of the remains of the Qin Great Wall and extended westward via what is now known as the Hexi (West of the Huanghe River) Corridor to Lop Nur in the Xinjiang Uygur Autonomous Region.

The section of the Great Wall west of the Huanghe River played a vital role in ensuring smooth traffic on the Silk Road to the western regions (mainly countries in Central Asia) and in the development of trade and cultural exchanges between China and countries in other parts of Asia and Europe. Like the walls built by many ducal states and dynasties, most of the Han Dynasty Great Wall gradually disappeared during ages of wind erosion and sand burial, leaving behind segments of earth or mounds of reeds and stone.

The Great Wall of the Ming Dynasty was the last to be built, and is the most perfect of the Great Walls in terms of defense engineering. Construction lasted for more than 200 years. The Great Wall that we see today is mainly the section completed during the Ming regime.

To achieve their goal of military defense, people in ancient China tried to make the design of the Great Wall as perfect as possible. The main body of the Great Wall extends for more than 5,000 kilometers, linking tens of thousands of block towers and beacon towers.

The beacon towers were also known as smoke mounds. They were built on the wall, on hilltops, or at spots easily seen on either side of the wall at regular intervals and were used as stations for military communications. If there was any enemy movement, a signal would be relayed—fire at night and smoke in the daytime—until the signal reached the capital or a large defense command post.

The block towers stood high above the wall and consisted of two or three tiers, including areas for the troops to live in and for storing weapons and ammunition. It is believed that the block towers were designed by Qi Jiguang, a general of the Ming Dynasty, who had distinguished himself in battles against Japanese pirates, and they played an important role in military defense.

The Great Walls were constructed with the rises and falls of China's feudal dynasties over a period of 2,700 years, and their remains can be found in the country's 16 provinces and autonomous regions in the northeast, north, and northwest, especially along the Huanghe River valley. The aggregate length of these walls was probably more than 50,000 kilometers. The masonry that went into the construction of the walls would be enough to erect a wall one meter thick and five meters high encircling the earth more than ten times. No other project in the world can boast such a huge amount of work in its making.

The Great Walls served as a monument to the political, economic, military, and cultural history of the feudal regimes as well as to the deeds of the bold, talented generals and the intelligent artisans.

The Walls also embodied the blood and sweat of numerous soldiers and laboring people. Legend has it that during the reign of the first emperor of the Qin Dynasty, a woman named Meng Jiangnu became distressed because her husband had not returned home for three years after being conscripted to work on the Great Wall. Meng decided to bring clothes to her husband. She suffered untold hardships before she arrived at Shanhaiguan Pass and wept in terrible grief when she learned that her husband had died of excessive labor and that his remains were buried under the Great Wall. Her tears caused 800 li (400 kilometers) of the Great Wall to collapse, and she found her husband's remains. The legend tells of the heavy forced labor over several thousands of years and the people's sufferings.

Today, the Great Wall stands as one of China's most well-known tourist attractions. People from all over the world put the Great Wall at the top of their list of places to visit when they come to China. The history, culture, art, and architecture of the Great Wall help them better understand China.

The Great Wall has lost its original appearance because of social changes and exposure to the weather over the past ages. After the founding of the People's Republic of China, measures were taken to protect it. In 1961, the Great Wall was designated as a major historical site under State protection. Maintenance and repairs have been conducted mainly at the sections at Badaling, Shanhaiguan Pass, and Jiayuguan Pass.

In 1987, The Great Wall was included in the List of World Heritages by the United Nations Educational, Scientific, and Cultural Organization (UNESCO). The Great Wall has become a treasure shared by all mankind.