A dig near Malmesbury town walls has uncovered a substantial stone-fronted defensive rampart and a deep ditch which could date to the Iron Age.
The fort would have had impressive multiple defences, experts said
Archaeologists believe the prehistoric hill fort would have had impressive multiple defences rising above the valley of the River Avon.
English Heritage said the results were very exciting and showed how important the town's defences were.
The work was said to bring a new dimension to the story of Malmesbury.
A project spokesman said it was the first time that the area outside of the line of defences has been examined archaeologically.
The finds add to discoveries recorded during the previous investigation carried out during November 2005 during restoration work on the walls, that revealed new evidence about the nature of the town's defences.
When the collapsing stone of the wall was removed, substantial clay deposits almost 3m (10ft) high were found. Archaeologists identified these as the upper rampart of the Iron Age hill fort on which Malmesbury was later built.
It is believed the whole of the Eastgate Bastion is an artificially constructed fortified gate (barbican) built to extend the area of the former hill fort and to provide substantial and impressive stone-built defences.
Investigations revealed evidence of a further rampart against the outer face of the lower levels of the town wall.
This consisted of burnt material including a large quantity of slag.
Archaeologists consider that this burnt material is probably Late Saxon and may date from the 8th or 9th Centuries AD.
If confirmed, it would add support to Malmesbury's claim to be the oldest borough in England.