More details have been revealed about the castle 'similar to the Tower of London' which lies beneath the basketball court at Gloucester prison.
The keep walls of the Norman castle have been found underneath the basketball court with other sections of the castle expected to be under other buildings at the site.
The castle dates back to 1110 and was found by archaeologists who are investigating the site ahead of a new development.
Chief Executive of Cotswold Archaeology, Neil Holbrook, 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."
The castle was the first in Gloucester to be built of stone housed three chapels, two drawbridges, a royal chamber for both the King and Queen.
Mr Holbrook said the design is thought to have resembled Canterbury castle and the Tower of London.
Mr Holbrook added: "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, which is 12 foot in width, was found just 60cm below the basketball court that would have been used by prisoners.
The keep itself is estimated to have been 30m long and 20m across, showing just how big this building was.
MP for Gloucester, Richard Graham, said: "It is a real treat to be lucky enough to see keep for the first time since 1790.
"The quality of the walls that they have uncovered is fantastic."
The dig site will be covered over today in the interest of protecting the area over the winter.
Mr Graham added: "We are still uncovering our history in Gloucester and this is another reminder of the rich heritage that the city has."
The purpose of the dig is to incorporate elements of the historical buildings with the new development. It is not yet decided what elements of the castle will be taken into account when constructing the new build.
Gloucester City Councils City Archaeologist, Andrew Armstrong, said: "It is wonderful; the quality of the remains is really outstanding.
"It is great to see these key parts of the castle so well preserved."
The castle had been used as the city jail for two hundred years prior to William Blackburn building his own prison on the site in 1780.
When this prison was built parts of the castle were damaged and destroyed but the keep wall remains largely intact despite the work.
Mr Armstrong added: "I didn't know what we would find but this is at the better end of what I would have expected."
City and Country now own the prison site and are considering what to do with the site and as a result of the dig finds they have altered plans to include elements of the sites history.
Richard Winsborough, Associate Director (Planning), City & Country, said: "Following these exciting discoveries, we have been working closely with Cotswold Archaeology, Gloucester City Council and Historic England to ensure the finds are appropriately considered before we continue with our plans."
He added: "As a result of this work taking place, we will now be hosting our third public consultation on the future use of the prison in the New Year and further details will be provided closer to the time."
