Benin City, the mighty medieval capital now lost without trace

Why is nothing left of what it once was one of the best-planned cities of the world? With its mathematical layout and earthworks longer than the Great Wall of China, Benin City was one of the best-planned cities in the world when London was a place of ‘thievery and murder’. So why is nothing left? Mawuna Koutonin for the Guardian provides us with an insight into this fascinating city.