A prehistoric fortress is home to a much later structure: what may be one of the biggest medieval palaces ever discovered, one whose remnants remain buried beneath the ground, the Independent reports.

The site in southern England is surrounded by huge earthworks that date to the Iron Age. Researchers used ground-penetrating radar and other technology to investigate what's under the grass within the inner and outer baileys of the former fort, the BBC reports.

Without doing any digging, they found a large complex that leading medieval-building expert Dr. Edward Impey believes is an early 12th-century castle. It measures about 560 feet by 210 feet and features 10-foot-thick walls and what appears to be a 200-foot-long great hall, the Independent notes.

"The prime candidate for constructing it is perhaps Henry I," says Impey. "Archaeologists and historians have known for centuries that there was a medieval city at Old Sarum," notes survey leader Kristian Strutt, "but until now there has been no proper plan of the site." The archaeologists' survey uncovered residential areas, evidence of kilns or furnaces, and an open area—"perhaps for mustering resources or people"—near some large structures, per a press release.

"From this we can piece together a detailed picture of the urban plan," says Strutt. The Iron Age fort at the site was likely built around 400 BC and taken over by Romans in 43 AD, the BBC notes.

But by the onset of the 13th century, the city built in the same place became too tight and weather-beaten for habitation and was abandoned in favor of today's Salisbury, which is located roughly 2 miles away.

(Another 'lost city' was recently investigated using similar techniques in Cambodia.)

This article originally appeared on Newser: Medieval Palace May Be Buried Underground

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