According to Wikipedia, The Walls of Benin were a series of earthworks made up of banks and ditches, called Iya in the local language. They extend for some 16,000 km in all, in a mosaic of more than 500 interconnected settlement boundaries. The Walls of Benin City were the world’s largest man-made earth structure. Fred Pearce wrote in New Scientist: They extend for some 160 km in all, in a mosaic of more than 500 interconnected settlement boundaries. They cover 2,510 sq. miles (6,500 square kilometres) and were all dug by the Edo people. In all, they are four times longer than the Great Wall of China and consumed a hundred times more material than the Great Pyramid of Cheops. They took an estimated 150 million hours of digging to construct, and are perhaps the largest single archaeological phenomenon on the planet.
Constructed over a period of 600 years, the Great Walls of Benin was located at the southern border of the defunct Benin Kingdom, which was one of the oldest and most highly developed states in West Africa.
It was brought down by the British in 1897.