Benin Iya

From NigerianWiki

The Benin Moat - or Iya, as it is termed locally - was originally constructed about 1400 years ago as a defense work surrounding Benin City, capital of one of the oldest kingdoms of the region. Built to an original height of more than 18 meters, and a length of 1,200 kilometers, the Iya was constructed in three stages. It was finalized around 1460, at that time being the world's second largest earthwork, after the Sungbo Eredo in neighboring Ijebu land which has been attributed to Queen Sheeba.

Source: http://www.afrol.com/articles/11299
http://csweb.bournemouth.ac.uk/africanlegacy/benin_Iya_earthworks.htm