Cambridge Castle, locally also known as Castle Mound, is in Cambridge, Cambridgeshire, England. Built after the Norman Conquest of England in 1066 to control the strategically important route to the north of England, it played a role in the conflicts of the Anarchy and the First and Second Barons' Wars. Hugely expanded by Edward I, the castle then fell rapidly into disuse in the late medieval era, its stonework robbed 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 jail. The castle jail 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
Eleventh 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 (Ermine Street) 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 18 or 27 houses had to be destroyed to make space for it.
Twelfth and thirteenth centuries
thumb|upright|Location of King's Ditch overlaid on a modern map
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. 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. 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. 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. More stonework was given away by Mary I in the 16th century for building a mansion at nearby Sawston in the Fens, and other grants of stone given to Emmanuel and Magdalene colleges. By 1604 only the gatehouse, used as a jail, and the keep remained intact, with the surrounding walls described by contemporaries as "rased and utterly ruinated". Oliver Cromwell ordered emergency work to be conducted to repair the defences, resulting in two new earthwork bastions 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. (equivalent to £ in ).
thumb|1841 painting of the castle gatehouse having lost its roof, and since demolished
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 and 1811 with an innovative octagonal structure, influenced by the designs of the prison reformer John Howard; the castle gatehouse was destroyed to make way for a new county court building.
By the end of the 19th century, the jail was deemed to be surplus to requirements as Huntingdon jail was sufficient to hold prisoners from both Huntingdonshire and Cambridgeshire. By 1920 the building had become a branch repository of the Public Record Office and remained so until 1928 when it was acquired by Cambridgeshire County Council. The jail was demolished in 1932 and the bricks used to build Shire Hall on the same site. Its only remains are the high motte (which stands on the highest point in the city) and some fragments of earthworks. It is open to the public daily with no admission fee, and offers views over the historic buildings of the city. The site of the castle bailey and the 19th-century prison is now occupied by Shire Hall, which was built in 1932 as the headquarters of Cambridgeshire County Council.