Some say the Empress Matilda has been seen on the castle's tower stairs
Britain's most seen ghost is the Empress Matilda, it has been claimed.
She allegedly haunts the stairs of the Oxford Castle in her trademark white cape.
Ghost hunting specialists, Fright Nights, discovered the ghost's popularity via feedback on sightings from their customer database.
"I had no idea how this compared to other haunted locations," admits Emily Hirons, manager of the Oxford Castle's visitor centre.
"Empress Matilda is our most witnessed ghostly figure... this is fantastic news!"
Matilda had taken up residence in Oxford Castle in 1141, but it was besieged by the forces of her cousin King Stephen.
She was left with no choice but to take flight in the dead of night. Stories recount how she was clad in a white cape to disguise herself against the snow, and then escaped along the frozen river.
The Anglo Saxon Chronicle describes how she was let down the tower by ropes. William of Malmesbury explained in his Historia Novella that she escaped with four knights by a postern gate and walked all the way to Abingdon.
At the time England was wracked by a civil war as the two cousins fought for their right to succeed Henry I, son of William the Conqueror.
Matilda was born in February 1102, and several historians agree that this would have been in Sutton Courtenay.
Dr George Garnett, University Lecturer in Modern History at St Hugh's College explains: "We know that her mother was there at the requisite time because she was issuing charts and writs in favour of the Abbot of Abingdon, Faritius.
"As well as being abbot he was the King's physician. It would make a lot of sense if she had gone to Sutton Courtenay for her confinement and was being attended by the abbot and making gifts to him.
Matilda became Empress after marrying the Holy Roman Emperor
"There's an undercroft in the manor house which is thought to be late 11th century. It's conceivable that that is the remains of some sort of residence where the Queen was staying."
Matilda was 11 years old when she married the European king Henry V, who was also the Holy Roman Emperor. Six years later her brother William, eldest son of the King of England, died in a shipwreck, leaving her next in line to the throne. When her husband died in 1125 she returned to England.
Soon after she was married to Geoffrey of Anjou, but they were in France when Henry I died, leaving her cousin Stephen free to seize the throne.
"In effect the kingdom was carved up into spheres of influence between 1139 and 1153," explains Dr Garnett. "It so happened that Matilda's support was strongest amongst the baronage in the west and Oxford was her eastern-most stronghold. That's why she spent so much time there."
Local historian Mark Davies, writer of Stories of Oxford Castle, elaborates: "It was a bit like pre-election fever, when people would go out canvassing. To win the city of Oxford round to her cause would be an important step."
Oxford was effectively becoming Matilda's centre of government, and it was there that she retreated when her plans for a coronation at Westminster Abbey fell through. The odds continued to stack up against the Empress when King Stephen laid siege to the castle.
The Oxford Castle was besieged by King Stephen in 1142
"It became clear that the garrison was going to have to surrender because they didn't have any food," says Dr Garnett. "That is when Matilda's famous escape took place, in December 1142."
Mark Davies describes the scene: "It was a December night, with the river frozen and the snow all around. Therefore she dressed in white clothing in order to be less visible to Stephen's forces.
"She was able to escape purely because the river was frozen. Getting down from St George's Tower would have been relatively easy. In normal times the river would have been as much a factor keeping people within the castle walls as it was at keeping them out of it."
There were several more years of unrest, including a pivotal confrontation in Wallingford between the armies of King Stephen and Henry, Duke of Normandy, who was Matilda's son.
The two forces faced each other off across the bridge but called a temporary truce. There followed the Treaty of Wallingford, which was apparently agreed between Henry and Stephen.
The agreement was confirmed in writing at Winchester, and later signed at Westminster. By this time Stephen's son Eustace had died, leaving Henry to become the next King of England upon Stephen's death.
Matilda lived to see her son take to the throne in 1154. Henry II of England was the first of the Plantagenet dynasty of monarchs, who ruled England for over 300 years.
Whilst the Empress fleeing across the snow in her white cape remains her most enduring image, Dr Garrett isn't convinced that her ghost still haunts the castle.
"I cannot conceive why it would be Matilda's ghost," he says. "She certainly didn't die there. She went on to live for decades. She died in the early 1160s in Normandy where she's buried."
It is there where her epitaph reads: "Great by Birth, Greater by Marriage, Greatest in her Offspring: Here lies Matilda, the daughter, wife, and mother of Henry."