Just 45 miles northwest of Beijing sits the longest man-made structure in the world, the Great Wall of China, a symbol of ancient civilizations that still stands. So just how long is it? The 5,500 miles of wall consists of trenches, hills and rivers built by dynasties beginning in 476 BC towering to defend against nomadic tribes from the north.

Different kingdoms initially built walls at strategic points to protect their territories, but in 22 BC, following the unification of China during the Qin Dynasty, the emperor decided to link and extend the walls.

Still, the wall itself is a series of discontinuous walls with spurs. Mongol invaders led by Genghis Khan could still maneuver around the wall and eventually conquered most of northern China between 1211 and 1223 AD, where they ruled until 1368 when the Ming defeated the Mongols.

Most of the wall as it stands now was rebuilt by the Ming Dynasty during the 16th century. About 1 million people, one-fifth of the population of China at the time, reconstructed the wall over 10 years. While folklore has it that the mortar used to bind the stones was made from human bones or that men are buried within the Great Wall to make it stronger, it’s mortar is made from rice flour. No bones have been found in any of the construction materials.

Watchtowers up to 40 feet in height were built at regular intervals. These lookout towers and fortresses housed troops and supplies. Defenders used beacons, smoke or flags to communicate. Workers often depicted Uranus, or Tianwang, the personification of Heaven, in reliefs found at strategic points on the Great Wall.

The Great Wall starts at Shanhaiguan Pass, a seaport along the coast of Bohai Bay, and travels on to Jiayuguan Pass in Gansu Province. At its highest point at Badaling it stands 2,624 feet above sea level. In most places, the wall is 25 feet high and ranges from 15 to 30 feet in width. The highest point of the Great Wall is in Beijing at Heita Mountain at 5,033 feet. The lowest point is at Laolongtou at sea level. The Great Wall’s western section and its chain of watchtowers provided defense for those traveling the Silk Road.

The most visited section of the Great Wall is in Badaling, close to Beijing, built during the Ming Dynasty. It first opened to tourists in 1957 and a visit by President Richard Nixon in 1972 spurred tourism. The section served as the finish site of a cycling course in the 2008 Summer Olympics.

Although some say you can see the Great Wall of China from the moon without visual aid, the factoid is a myth started in 1893 in a magazine, The Century, and later resurfaced in 1932 in Ripley’s Believe it Or Not.