It was a scholarly investigation by Leslie Alcock and David Cathcart King in the 1960's that first suggested the earthwork could be the remains of a medieval siege work. Prior to this John Aubrey, the 17th century antiquary, had written of an ancient mound above Cole Park; while the Victoria County History describes circular and rectangular enclosures near Cam's Hill as prehistoric. A recent investigation by Exeter University supports the siege castle theory.

The Anarchy coincided with the reign of King Stephen, 1135 -1154. During this period there was almost 20 years of chaos, civil war and lack of governance in England. This was fundamentally a war of succession between two cousins - Stephen and Matilda - who both claimed the throne of England after the death of Henry I. Malmesbury Castle was a thorn in Matilda’s side. While much of the south west of England was controlled by her forces during the Anarchy, the garrison at Malmesbury remained stubbornly loyal to Stephen. In 1153 Matilda’s son, the future King Henry II finally broke the deadlock. He came to Malmesbury, successfully confronted Stephen and his army, and took control of the castle. A year later he was crowned king of England.

The castle had been built just a few years before. Early in the 12th century Malmesbury Abbey had come under the control of Bishop Roger of Salisbury and he established a castle close to the abbey buildings. In 1139 King Stephen was concerned about the loyalty of Roger and some other bishops and seized their castles, including Malmesbury. Civil war broke out shortly afterwards between Stephen and his rival claimant for the throne, the Empress Matilda, in which Malmesbury castle was at times a focus.

At the start of the fighting in 1139, a Flemish mercenary captain called Robert Fitz Hubert seized Malmesbury Castle from Stephen, sacking the town in the process. King Stephen came to Malmesbury in person and recaptured the castle shortly afterwards. The castle was then used by royal forces as a base for raiding those parts of the surrounding countryside that were loyal to Matilda.

The chronicle, Gesta Stephani, described the siege of 1144 when the Earl of Gloucester built three siege castles around the town in order to starve the Malmesbury garrison into submission. The Exeter study suggests that it was most likely at this time that the fortification of Cam's Hill was erected. The chronicle explained that King Stephen himself raced to Malmesbury again in person to relieve this siege: "When the king learnt the truth about his men's grievous plight he at once called a vast army together and arrived at Malmesbury suddenly and unexpectedly."

Malmesbury castle then remained in royal hands until taken by the future Henry II in 1153 towards the end of the civil war. In the reign of King John the Malmesbury monks petitioned to have the castle destroyed; the king agreed in 1216 and it was demolished in the early 13th century.

Today it is thought that no remains of the castle survive, and archaeology has yet to identify the site of the castle within the town.

Exeter University's investigations of the Cam's Hill site were a part of their research project 'Anarchy? War and Status in Twelfth Century Landscapes of Conflict'. The geophysical and topological investigation included magnetometer and resistance surveys and an earthwork survey. The Exeter University paper fully describing and discussing this survey is published in Wiltshire Archaeological and Natural History Magazine, Vol. 108 (2015), pp. 105 - 18.

These findings suggest firstly that there is a C - shaped enclosure with a bank and outer ditch with a large opening to the north, and that this enclosure may have been raised over a springhead (the springhead theory may explain why this site just below the highest point was chosen). And secondly that there is a hollow way on the eastern side. At some time the entrance way may have been restricted. An anomaly seen on the resistivity survey might represent the foundations of an observation tower. The Exeter team endorsed the idea that this structure was, perhaps, part of the siege castle constructed by Robert of Gloucester in 1144 as he sought to control access to the castle and town.