In and around 500 BC, the Qi Dynasty was at war against other powerful states. To protect itself, construction began on what would go on to become a massive wall 370 miles long. This wall wasn't stone and mortar, like the Great Wall that would follow—much of the Qi wall was made of rammed earth, says Gary Feinman for the Field Museum:
[W]orkers took great pains to carry huge amounts of fine soil to elevated areas, and then vigorously pounded the sediment (mixed with water and perhaps a binding agent) to heights of 15 feet, in some places!
“This was a major effort, because the wall follows ridges into mountainous areas that would have been very difficult to reach with great volumes of earth,” Feinman said. Today, the wall is best preserved at these higher elevations where it likely was most difficult to erect, but where it is not endangered by subsequent farming and flooding.
While it could be scaled with ropes, the Qi wall would have been an effective impediment against large infantries, which likely had accompanying carts carrying food and supplies. Such massive arrays of foot soldiers would have been slowed down as ropes were deployed to scale the wall, and some supplies would have likely been lost or left behind.
In the ancient world walls were a common military tool, but the aside from the Great Wall of China the Qi wall was the largest, beating out the next largest wall, the Great Wall of Gorgan in Iran, by nearly 250 miles. Even in the modern world the Qi wall would stand tall—that is, if it weren't being torn apart or used as a road.