Archaeologists have discovered a previously unknown section of Chester Castle's medieval defences.

Professor Stewart Ainsworth, Time Team's landscape archaeologist and a visiting professor at the University of Chester, made the discovery as he explored the remains of the 18th-century prison beneath Chester Crown Court.

While investigating an underground corridor, Professor Ainsworth spotted a section of stonework that looked very different from the surrounding masonry. He said he immediately recognised the stonework as medieval after comparing it with surviving parts of the Norman castle.

"When I compared the type of stones and the building technique with the surviving parts of the Norman castle, it was clear that this underground wall must also be medieval," he said.

"The wall I discovered is massive – almost three metres high – so we now know that he left a lot of the medieval stonework untouched. And I'm guessing much more may survive under the car park and castle complex."

High-resolution digital scanners were then used to pinpoint the wall's location.

Comparisons with historic maps and plans revealed it once formed part of a medieval bridge spanning the deep defensive ditch around the castle's inner ward.

The bridge is thought to have been built around 800 years ago, between the 1260s and 1280s, when a heavily fortified gateway replaced an earlier entrance. Although Thomas Harrison later demolished that 13th-century gateway, the newly discovered wall shows significant parts of the medieval structure survived.

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The castle's original stone gateway, now known as the Agricola Tower, is cared for by English Heritage and is open to visitors at weekends. On the first floor, the Chapel of St Mary de Castro contains fragments of rare wall paintings dating from the 1240s.

Researchers say the newly discovered wall closely resembles part of the bridge at Beeston Castle, where the remains of a medieval drawbridge can still be seen. They believe Chester Castle may once have had a similar defensive arrangement.

The discovery was made as part of the Digital Dimensions project, which has seen the underground remains and other parts of the castle digitally scanned to create highly accurate 3D models.

Led by the University of Chester and Cheshire Constabulary, alongside partners including English Heritage, Historic England, SCCS North and the Hartree National Centre for Digital Innovation, the project is testing digital recording techniques to help tackle the theft and damage of historic stonework.