Work to repair the mound at Oxford Castle has revealed a 10-sided tower that has been hidden for more than 200 years.

The foundations of the 13th century tower which used to stand on top of the mound were discovered during repairs following last year's landslide, when part of the mound collapsed into New Road.

The mound is the main surviving part of the Norman castle built by Robert D'Oilly in the 1070s.

It was originally crowned by a timber tower, inside a defensive palisade, reached by a bridge from the courtyard below.

But the mound was raised in height during the 13th century, when the stone tower was built overlooking the medieval city. It is a section of the outside wall that has now been exposed, to the delight of archaeologists.

Greg Lowe, who is supervising the project for Oxfordshire County Council, said: "The emergence of the base of the tower is fascinating. Oxford Archaeology has been analysing the structure and will report their full findings when the work is complete.

"This is a postscript that we didn't expect.

"The foundations were last exposed more than 200 years ago, by the then keeper of the prison, who was a keen archaeologist.

"Since then it has been very much supposition. We didn't know how close they were to the surface or anything about the configuration of the tower."

"There's a little more work to do on the site than we had originally anticipated, so the work will go on for slightly longer.

"This is not something that could have been planned for until we had actually cut into the mound and put ourselves in a position to do a more detailed assessment."

Arrangements are now being made to allow visitors to see the new discovery, which may be covered up again once work is complete.

Debbie Dance, director of the Oxford Preservation Trust, which spearheaded the restoration of adjoining buildings which now house the Oxford Castle Unlocked visitor attraction, said: "This is a hugely exciting discovery.

"We're keen to be able to share this discovery with everyone and are working with the council to make this happen.

"But at present it looks as though the foundations will only be open temporarily and may have to be covered again."

Restoration work on the mound will continue until the end of July - six weeks longer than was originally anticipated.

After workmen began to dig into the earthwork it was discovered that there are gaps between soil layers in areas near to where the work is taking place, increasing the risk of further slippage if not repaired.

Special access arrangements, allowing members of the public to see the tower foundations, are being put in place for the weekend of Saturday, May 31.