A Norman castle, similar to the Tower of London, has been found under Gloucester Prison.
Resembling the famous London fortress and Canterbury Castle, the building dates back to 1110, and lay just 60cm below the ground on which prisoners used to play basketball.
The facility was closed in 2013, and plans for redevelopment of the Victorian prison meant archaeologists were brought in to investigate the site.
The castle was made of stone, the first stone building in the city, and included three chapels, two drawbridges, and a Royal chamber for both the King and Queen. The keep alone was roughly 30m long and 20m wide, showing the scale of the fortress.
From the late 1500s, the castle had been used as the city jail. In 1780, William Blackburn built his own prison on the site.
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.”
New plans for the prison want to incorporate elements of the historical buildings with the new development, though there are no firm plans as of yet.
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.”