Cambridge Castle Cambridge Castle , locally also known as Castle Mound , is located in Cambridge , Cambridgeshire , England . Originally built after the Norman conquest to control the strategically important route to the north of England , it played a role in the conflicts of the Anarchy , the First and Second Barons ' War s . Hugely expanded by Edward I , the castle then fell rapidly into disuse in the late medieval era , its stonework recycled for building purposes in the surrounding colleges . Cambridge Castle was refortified during the English Civil War but once again fell into disuse , used primarily as the county gaol . The castle gaol was finally demolished in 1842 , with a new prison built in the castle bailey . This prison was demolished in 1932 , replaced with the modern Shire Hall , and only the castle motte and limited earthworks still stand . The site is open to the public daily and offers views over the historic buildings of the city . History . 11th century . Cambridge Castle was one of three castles built across the east of England in late 1068 by William the Conqueror in the aftermath of his northern campaign to capture York . Cambridge , or Grantabridge as it was then known , was on the old Roman route from London to York and was both strategically significant and at risk of rebellion . The initial building work was conducted by Picot , the high sheriff , who later founded a priory beside the castle . The castle was built in a motte and bailey design , within the existing town , and 27 houses had to be destroyed to make space for it . 12th - 13th centuries . The castle was held by the Norman kings until the civil war of the Anarchy broke out in 1139 . Castles played a key role in the conflict between the Empress Matilda and King Stephen , and in 1143 Geoffrey de Mandeville , a supporter of the Empress , attacked Cambridge ; the town was raided and the castle temporarily captured . Stephen responded with a counter - attack , forcing Geoffrey to retreat into the Fens and retaking the castle . Cambridge Castle remained exposed , however , and Stephen decided to build a supporting fortification at Burwell to provide additional protection . Geoffrey died attacking Burwell Castle the following year , leaving Cambridge Castle secure . Under Henry II the castle was adequately maintained , but little additional work was undertaken to improve it . A castle - guard system was established , under which lands around Cambridge were granted to local lords on the condition that they provide guard forces for the castle , and the castle was primarily used to hold the sheriff ' s court and records . King John expanded the castle in the years before the First Barons ' War of 1215 to 1217 , but this work was concentrated on constructing a new hall and chamber , at the cost of £200 . During the war , the rebel barons , supported by Prince Louis of France , captured much of eastern England ; Cambridge Castle fell in 1216 . The castle was returned to royal control after the war , but Henry III only conducted basic maintenance of the fortification . Cambridge was attacked again during the Second Barons ' War in 1266 . This time the town and castle held long enough to be relieved by Henry ' s forces , but the king reinforced the city defences with a large ditch , later known as King ' s Ditch . Cambridge Castle remained only a basic fortification until 1284 when Edward I decided to undertake major expansion works . Over the next 14 years the king spent at least £2 , 630 on rebuilding the castle in stone . Edward ' s castle was four - sided , with circular towers at each corner , guarded by a gatehouse and a barbican . A circular stone keep was built on the motte . The result was a " major fortress in the latest fashion " , albeit never quite completed . Edward stayed at the castle for two nights in 1294 . 14th - 17th centuries . During the 14th century the castle was allowed to fall into disrepair . From Edward III onwards , little money was spent on maintaining the property and by the 15th century the castle was in ruins . The castle hall and chamber were roofless by the 15th century , and Henry VI ordered these buildings to be destroyed and the stone reused for constructing King ' s College in 1441 , with other parts of the castle being used to help build Trinity College ' s chapel . More stonework was given away by Mary I in the 16th century for building a mansion at nearby Scawston in the Fens , and other grants of stone given to Emmanuel and Magdalene colleges . By 1604 only the gatehouse , used as a gaol , and the keep remained intact , with the surrounding walls described by contemporaries as " rased and utterly ruinated " . Civil war broke out in England in 1642 between the rival factions of the Royalists and Parliament . Cambridge Castle was occupied by Parliamentary forces in the first year of the war . Oliver Cromwell ordered emergency work to be conducted to repair the defences , resulting in two new earthwork bastion s being added to the castle and a brick barracks constructed in the old bailey . The governor of Cambridge described in 1643 that " our town and castle are now very strongly fortified . . . with breastworks and bulwarks " . The castle saw no further fighting during the war , and in 1647 parliament ordered the remaining fortifications to be slighted , damaged beyond further use . 18th - 19th centuries . The castle rapidly deteriorated after the slighting and the remaining walls and bastions were taken down in 1785 , leaving only the gatehouse and the earth motte . The gatehouse remained in use as the county gaol into the 19th century , being run , like other similar prisons , as a private business — the keeper of the castle gaol was paid £200 ( equivalent to £13 , 100 in 2009 prices ) a year by the county in 1807 . This came to an end when a new county prison was built in the grounds of the castle ' s former bailey . The new prison was built by G . Byfield between 1807 to 1811 with an innovative octagonal structure , influenced by the designs of the prison reformer John Howard ; the castle gatehouse was then destroyed to make way for a new county court building . Today . The only remaining parts of the medieval castle left today is the 10 metre ( 33 feet ) high motte , which rests on the highest point in the city , and some fragments of the surrounding earthworks . It is open to the public daily with no admission fee , and offers views over the historic buildings of the city . Both the motte and earthworks are Scheduled Ancient Monuments . The site of the castle bailey and the 19th century prison is now occupied by the Cambridgeshire County Council ' s headquarters at Shire Hall , built in 1932 .