When archaeologists undertook the task to evaluate the site of an old prison in Gloucester, England, they had no idea they would find such well-preserved castle ruins.

The prison's complex featured buildings , while the site itself had been a jail for more than 500 years, Cotswold Archaeology said in a press release. Archaeologists revealed a 12th-century castle keep wall beneath the considerably more modern recreation yard.

"The castle was a large structure, with the keep, which we have now located in our work, an inner bailey and stables," the press release stated. "The keep was surrounded by a series of concentric defences which comprised curtain walls and ditches, with the drawbridge and gatehouse lying outside to the north."

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It was a structure so impressive that archaeologists have compared it to the Tower of London.

"It would have been a ," Neil Holbrook, who led the excavation, told Gloucester Citizen. "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 castle keep, which dates back to 1110, was about 100 feet long and 65 feet wide. The wall itself, 12 feet wide, defied expectations by surviving centuries in its remarkable condition underground.

Archaeologists also uncovered hundreds of artifacts, from pottery to medieval bone die.

The site will be covered to withstand the winter while developers decide what to do with the property.

According to the Daily Mail, the site owners City and Country Group will run a – though they don't know how just yet, they plan to incorporate elements of the site's incredible history.