Over 2,000 years old, the Great Wall of China remains one of the great wonders of the world, an engineering feat rarely matched in the 22 centuries since its construction began. Stretching 4,500 miles, from the mountains of Korea to the Gobi Desert, it was first built to protect an ancient Chinese empire from marauding tribes from the north. But it evolved into something far greater, a boon to trade and prosperity and ultimately a symbol of Chinese ingenuity and will.

The Great Wall is actually a series of walls built and rebuilt by different dynasties over 1,000 years. And while they often served the same purpose, these walls reflected the worlds, both natural and cultural, in which they were erected. The Great Wall is an emblem of China's evolution.

The project began under the tyranny of Qin Shihuangdi in the Qin Dynasty, in 221BC. Older, ruined walls were already there, but were broken into separate pieces here and there in Northern China. With both forced and conscripted labor, the old walls were restored and a new, continuous wall created. The goal was a 3,000-mile-long fortification meant to protect China's northern frontiers against attack by marauding nomads. Much of the forced labor came from exiled scholars and philosophers, most of them died there. This made the Qin Emperor extremely unpopular.

With the fall of the Qin Dynasty, the wall fell into centuries of neglect. China's northern frontiers were at the mercy of fierce outsiders known as the Xiongnu. After 70 years of war, the Xiongnu were crushed once and for all. With this victory by Han Wu-Di, the greatest of the Han Dynasty emperors, came a westward expansion into the wilderness of Central Asia. To protect that border Wu-Di began China's second great campaign of wall building. His engineers restored the crumbling Qin wall and extended it 300 more miles across the forbidding Gobi Desert. With Central Asia under Han control, safe caravan routes - the legendary Silk Roads - were established, opening China to the commerce and culture of the Western world. Traders from Rome, Antioch, Baghdad and Alexandria trekked eastward to deal in jade, gold, spices, horses, precious gems and of course, silk.

The Han also added beacon towers to the Wall, spaced 15 to 30 miles apart. Columns of smoke were used to warn defenders of an attack. One smoke column meant an outpost was being threatened by a force of fewer than 500 troops, two columns meant an attacking force of fewer than 3,000.

The greatest of all wall builders were the ones in the Ming Dynasty, whose astounding accomplishments dwarfed what had been done earlier by the Qin and the Han. The Ming not only built more wall than any other dynasty, but theirs was also bigger, longer, more ornate and more imposing. Coming out of the Mongol occupation (Yuan Dynasty), the Chinese were eager to protect themselves from further attack from the Northern tribes.

The Ming wall was built of a tamped earth interior encased in a facing of kiln-fired brick, pushing wall-building technologies to the limit. The Ming wall snakes acrobatically across some of China's most forbidding terrain, rising in places at an angle of 70 degrees. Such dramatic engineering wouldn't have been possible without the Ming's advanced brick-making technology. At a time when European builders were still relying upon cumbersome cut stone, the Ming were using state-of-the-art kilns to mass produce bricks, which were as strong as modern-day masonry blocks.

In recent years, the Great Wall has been slowly chipped away by people who need stones to build houses. As useless as the Great Wall is now in modern times, it is still a symbol of pride in China. Today, old, crumbling sections of wall are allowed to be dismantled, while the newer walls, built in the Ming Dynasty, are kept in condition, because it is a national relic, as well as bringing in plenty of (of course) tourist money.