The Great Wall of China is the longest wall in the world. The Great Wall of China is not a continuous wall but is a collection of short walls. These short walls were being built as early as the 7th century BCE, and later joined together, are now collectively referred to as the Great Wall. It is made of stone, brick, tamped earth, wood, and other materials. The wall with all of its branches, extends about 8,850 km. This is made up of 6,259.6 km sections of actual wall, 359.7 km of trenches and 2,232.5 km of natural defensive barriers such as hills and rivers.
By the time of the Spring and Autumn period between the 8th and 5th centuries BC, the Chinese were already familiar with the techniques of wall-building. Northern china was ruled by the Liao, Jin and Yuan dynasties throughout most of the 10th-13th centuries. They constructed defensive walls in the 12th century within china's province of inner Mongolia. These were located much to the north of the great wall of china of today. The Ming revived the concept of Great wall of china in the 14th century. They constructed walls along the northern border of china to keep nomadic tribes out under their new strategy. Up to 25,000 watchtowers are estimated to have been constructed on the wall. The Great Wall helped defend the empire against the Manchu invasions that began around 1600, towards the end of the Ming. Some additions and modifications were made to these simple walls over the next millennium
The Great Wall of China was mainly built from rammed earth, stones and wood before bricks were used in the construction. However, during the rule of Ming, materials such as tiles, lime and stone as well as bricks were used. The construction of wall quickened with bricks as the size and weight of bricks made them easier to work with. Stone too were used in the foundation, inner and outer brims and at the gateways of the wall after being cut in rectangular shapes. Barracks, stables, and armories were built near the wall's inner surface.
Visibility from space : The claim the Great Wall is visible has been debunked many times. Unsurprisingly, no lunar astronaut has ever claimed to have seen the Great Wall from the moon. From a low orbit of the earth, many artificial objects are visible on the earth, such as highways, ships in the sea, railroads, cities, fields of crops, and even some individual buildings. While at a low orbit, the Great Wall of China can certainly be seen from space but it is not unique in that regard.