Evidence has revealed the town of Venta Icenorum at Caistor St Edmund, near Norwich, extends "substantially" beyond the current archaeological site.
Over the weekend, experts are digging test pits to determine the extent of Roman occupation in the area.
It is hoped the work will reveal more about life there before the Romans.
Dr Will Bowden, from the University of Nottingham, which is working with the Norfolk Archaeological Trust, has specialised in the site for the last eight years.
He said: "Hopefully we'll get clues as to what was on the site before the Roman town was built.
"Our surveys have shown the defensive ditches, which appear in the shape of a kite, enclose a much larger area than is covered by the street grid of the Roman town.
"The ditches are quite early, earlier than some of the Roman streets, so it is possible that they reflect an early ambitious plan for the town that never worked out."
Dr Bowden said the layout of ditches in the area suggested there was "something worth defending in that area".
The Romans abandoned Venta Icenorum in the early 5th Century.
About a dozen volunteers will be working on the dig funded by a £10,000 grant from the Heritage Lottery Fund.
The project will also examine the history of Caistor village to see how it developed after the Roman period until the present day.
"This is not just about the town but the whole landscape and getting people involved with archaeology by exploring what's underneath their house," said Dr Bowden.
Source: BBC News: http://www.bbc.com/news/uk-england-norfolk-28867985