Walls of Benin
The Walls of Benin was a combination of ramparts and moats, called Iya, used as a defense of the capital Benin City in present-day Edo State of Nigeria. It was considered the largest man-made structure lengthwise, second only to the Great Wall of China and the largest earthwork in the world. With more recent work by Patrick Darling, it has been established as the largest man-made structure in the world, larger than Sungbo’s Eredo. It enclosed 6,500 km² of community lands. Its length was over 16,000 km of earth boundaries. It was estimated that earliest construction began in 800 AD and continued into the mid 1400’s.
That picture isn’t the walls of Benin though, and their just run of the mill stockades, while the walls of Benin were massive fortifications. They stretched for miles, like in the map below.
Maps from [here] [But Benin isn’t a “forgotten kingdom” though].