The vast complex - the most extensive earthwork system in the world - covers 2,500 square miles and consists of more than 500 interconnected communal enclosures, according to survey work being carried out by Dr Patrick Darling.
Using archaeological, historical and linguistic evidence, he estimates that most of the complex was built progressively over a 450-650 year period - from between 800 and 1000AD up until the late 15th century, when much of the area was conquered by a local African power later known as the Benin Empire.
Many of the communal enclosures are around 20 miles in circumference - although some are much larger, up to 70 miles long.
On average, the ramparts tend to be around 10 feet high (from bottom of ditch to top of bank) - although some rise to around 60 feet in height. So far Dr Darling - now funded by the British Academy - has made a detailed plan of around 10 per cent of the complex.
The precise reason why medieval West Africans built the earthworks remains a mystery. However, Dr Darling is developing a series of possible explanations.
He thinks it probable that the earthworks functioned as communal boundaries, delineating the agricultural land belonging to local extended families and lineage groups.
He therefore believes that the boundaries defined communities of related individuals who were not allowed to intermarry.
Men had to continue living within the ramparts after marriage, while the community's womenfolk were obliged to marry men from beyond the ramparts, and thus move to other enclosures.
The earthworks - which are estimated to have taken 150 million man-hours to build - also probably functioned as a communal status symbol. Dr Darling's survey has revealed that the ramparts become more impressive closer to entranceways.
He also suspects that the banks and ditches may at some stage have acted, in ritual terms, as a symbolic boundary between the real world and the spirit one.
Historical research has revealed that hundreds of years ago the corpses of childless men were placed in the boundary ditches - literally between this world and the next.
The ramparts appear to have been built by a West African people known as the Edo - initially either after emigrating from grassland areas in central Nigeria, or in response to settlement pressure from Ibo immigrants from the east.
Although most of the earthwork construction activity ceased with the Benin conquest in about 1480, some communities fled to other areas and continued the tradition there. Around 30 per cent of the earthworks fall into this category.
But the rulers of the Benin Empire also appear to have adapted the earthwork concept for their own use. Around every 10 miles along the major trade routes, the Benin Emperors (known as Obas) appear to have built small earthen enclosures.
Strangely, these have their ditches on the inside of their banks rather than on the outside, and it is likely that they were used by the Benin imperial government to house batches of slaves after a day's march.
Much of Benin's economy and security was based on taking people from neighbouring areas against their will, and forcing them into slavery - known as 'slave raiding'. For a period, the Obas banned the sale of slaves to Europeans - because they wanted them themselves.
The empire used slave raiding to increase its population, expand existing communities, build up its army, and to settle new agricultural land, thus producing increased tribute for the emperor.
These slaves were actually known euphemistically as 'children of the Oba', and their introduction into society on a large scale destroyed the traditional lineage-based system and reinforced central control. The Oba ruled from his palace in Benin City, which was then the capital of the empire.
However, Dr Darling's study of the city's vast communal earthworks suggests that they date from before the birth of the Benin kingdom from which the empire evolved.
New examinations of the 60-feet-high Benin ramparts show that they were merely a part of the original early earthwork system, which suggests that an important political and religious centre developed there before it was conquered by the founders of the Benin kingdom and empire.
The Benin civilisation is included as an option in the primary school history syllabus within the National Curriculum. A display of items about Benin is being shown at the Museum of Mankind, Burlington Gardens, London W1. Admission free.
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