The skeleton of a man, known as 180, was found in a medieval cemetery when Western Road School was demolished in 1994.
Radiocarbon dating places the man’s death to within 28 years of 1063. He could therefore have been involved in the battles associated with the Norman invasion in 1066.
He could even have been killed during the actual Battle of Hastings.
The cause of death was six sword injuries inflicted on the back of the skull.
He ate a diet particularly rich in marine fish and was at least 45 years old but may have been older. He had some spinal abnormalities and suffered from chronic infection of the sinuses.
The first was probably a cut to the right side of the ear and upper jaw followed by a series of sword cuts, all delivered from the left hand side behind the victim.
The site where the skeleton was found was once the home of the Hospital of St Nicholas, which was run in medieval times by monks from Lewes Priory.
The hospital is thought to have been at the centre of the fighting in the Battle of Lewes in 1264, and as the man had died violently it was assumed that was when he was killed.
However, when the Sussex Archaeological Society sent the remains to the University of York for carbon dating to tie in with the Battle of Lewes celebrations last month, the surprising discovery was made.