Archaeologists unearthed the enormous ‘tower keep’, which is thought to date back to 1110, while they were investigating the site in Gloucester ahead of a redevelopment project.
The castle, which was among the first buildings to be built of stone, was thought to have been destroyed in 1787 and the Gloucester prison built on top.
Neil Holbrook, chief executive of Cotswold Archaeology who led the excavation, said: ‘It would have been a powerful symbol of Norman architecture.
‘As you came to Gloucester you would have seen the cathedral and the castle, which is representative of how important the city was in Norman Britain.’
The keep wall is more than 12 feet wide and the building is estimated to have been 98 feet (30m) long and 65 feet (20m) across.
It housed three chapels, two drawbridges and a royal chamber for both the King and Queen.
The building is thought to have served as the dominant fortress in the Gloucester area after it was built before being used as the city jail for 200 years ahead of its destruction in 1780.
It appears, however, rather than completely remove the structure, the new prison was merely built on top of it.
Neil Holbrook, chief executive of Cotswold Archaeology, said: ‘I am surprised by what we found.
‘I knew there was a castle but I had expected more of it to have been destroyed.’
Photo: Andrew Higgins/SWNS