Construction on the fort began around 800BC and took centuries to complete
The full extent of a hill fort likened to an Iron Age "Millennium Stadium" has been uncovered by investigators.
Gaer Fawr hillfort at Guilsfield, near Welshpool, Powys, is effectively hidden by woodland, making it impossible to appreciate the scale of it.
Detailed survey by the Royal Commission on the Ancient and Historical Monuments of Wales produced a computer model of the site which dates from around 800BC.
The fort features in the BBC Wales programme Hidden Histories on Tuesday.
Gaer Fawr is one of several hillforts overlooking the upper Severn valley.
The forest covers 5.8 hectares (21 acres) with up to five lines of ramparts over 8m (26ft) high and is now covered in woods.
The Royal Commission's digital terrain model allows the hillfort and its setting to be visualised and makes it clear that Gaer Fawr was constructed in many phases.
It originated as a small summit-fort and additional space was enclosed later as well as two highly-developed entrance-ways created with graded approach ramps and defended gateways.
An annexe was also added to the south.
One feature uncovered in the survey is a bank dividing the interior space, possibly associated with early medieval occupation.
Building work on the fort probably began around 800BC and would have continued for about four centuries.
Dr Toby Driver, aerial investigator for the Royal Commission, told BBC Wales' news website many of the people who visited the fort site, which is owned by the Woodland Trust, would have been unaware what the "lumps and bumps they were walking on" actually were.
He said the construction and scale of the site made it comparable to a modern edifice like the Millennium Stadium in Cardiff.
"The timber required alone for the ramparts would have stripped woodland for miles around so it gives an idea of the land it controlled.
"It would have been an unusual site and quite outstanding. After a few centuries of building this site, people who controlled it would have hoped it would be quite terrifying.
"It's tricky to estimate population - the best analogy is that it is like a small town.
"Maybe a few hundred people would have been living there.
"It would have had a chiefly residence and would have run markets monthly, and would have had farms surrounding it."
He added it took surveyor Louise Barker about three weeks to complete the physical survey of the area, which was then converted to a computer model by digital mapper Tom Peat.
Hidden Histories is broadcast on BBC2 Wales on Tuesday 11 November at 1930 GMT.
The computer model was based on three weeks of on-the-ground survey work