The Great Pyramid is certainly impressive by any standard, especially given the technology available at the time. But from the standpoints of engineering, logistics, project management, sheer grandiosity, and persistence, nothing on earth really compares to the Great Wall. It is hard to believe that human beings could ever be organized long enough and well enough to engineer and build something of this scale. Imagine that we decided to build a wall of stone and brick 20 feet (6 meters) wide and 20 feet high, along with substantially larger watch towers every 1,000 feet (300 meters) or so, from Washington DC to Los Angeles (a distance of approximately 2,600 miles or 4,200 km) and then back again. That is the scale of the Great Wall. By volume of material, the Great Wall might be 100 times larger than the Great Pyramid. It is difficult to know for sure because large sections of the wall have eroded, collapsed, or been dismantled The Great Wall, completed in 1600, was built to solve a problem. Various nomadic groups of non-Chinese people, including the Mongols and the Manchus, were trying to invade China. The wall was meant to provide a line of demarcation and defense. It was built over a long period of time - 1,000 years or more. The Ming Dynasty, from 1368 through 1644, stands out as a time when the 5,500 miles (8,860 km) was consolidated, linked, reinforced, and standardized to create the wall we know today. The basic architecture for a wall section is fairly straightforward. Two thick stone or brick walls were built 20 feet apart, and then the gap filled with dirt and rubble. Paving stones or bricks along the top of the wall covered the dirt fill to create a path approximately 17 feet (5 meters) wide. With anything we build today, we have assistance from machines — bulldozers, tunnel boring machines, tower cranes, dump trucks. The Great Wall will stand out as a singular achievement because, despite its scale, it was built instead by millions of hands. >Several walls within what became the Great Wall were being built as early as the seventh century BCE.