Answer: Nigeria

When it comes to walls, the Great Wall of China gets all the attention. The Great Wall is, without a doubt, quite a structure and thanks to the materials from which it was constructed, much of it still exists today (centuries after the fact).

But the Great Wall isn’t the longest continuous wall ever built, being approximately 4,000 miles (6,437 kilometers) long in its longest unbroken section (a bit of construction dating from the Ming dynasty). The longest continuous wall ever built was actually the work of an African kingdom in what is now present day Nigeria. Between the first millennium C.E. and the mid-fifteenth century C.E., workers in the Kingdom of Benin built a massive moat and rampart earthwork wall (they excavated a huge moat and used the earth to build a very thick packed mud wall). This wall enclosed the heart of the kingdom, approximately 2,500 square miles (6,500 square kilometers) and had a length of approximately 9,900 miles (16,000 kilometers).

Unfortunately, like most ancient structures in Africa, the wall was earthen and after the fall of the Benin empire (and the subsequent lack of maintenance), seasonal rains and erosion quickly took their toll. While sections of the wall remain today, it has been over two centuries since the wall was in its prime glory.