Following various minor forts during the Roman era, Bristol's first castle was a timber motte and bailey, built under Norman rule. Its construction was doubtless ordered by William the Conqueror, who owned Bristol.
By the 12th century the original structure was replaced by a stone castle (along with many others from the first wave of Norman timber castles throughout England). The new Bristol Castle, built by Robert of Gloucester, had new curtain walls, a great keep, and was positioned on a strategic site to the east of the town, between the River Avon and the River Frome. The latter was partly diverted to form the castle moat.
This castle was one of the strongest in southern England, and, according to Stephen of Blois's chronicler in 1138, made Bristol impenetrable: "On one side of it [Bristol], where it is considered more exposed to siege and more accessible, a castle rising on a vast mound, strengthened by wall and battlements, towers and divers engines, prevents an enemy's approach."
Bristol Castle was later taken into royal hands. During the 13th century Henry III spent lavishly on it, adding a barbican before the main west gate, a gate tower and magnificent great hall.
By the 16th century the castle was showing signs of neglect, and it fell into disuse in the late Tudor era. However, the city authorities had no control over royal property, and the precincts became a refuge for criminals.
In 1630 the city bought the castle, and when the Civil War broke out the city took the Parliamentary side and partly restored the fort. However, Royalist troops occupied Bristol, and Oliver Cromwell eventually ordered the destruction of the castle in 1656. One octagonal tower survived until it was torn down in 1927, and some remains of the banqueting hall have been integrated into an existing building.
The castle moat was covered over in 1847, though this still exists and is mainly navigable by boat, flowing under Castle Park and into the Floating Harbour. The western section is now a dry ditch, and a sally port into the moat survives near St Peter's Church.
The castle area was eventually redeveloped for commerce. This district was largely destroyed in the 'Bristol Blitz' of World War II, and was subsequently revived as a public open space, Castle Park. The castle remains can still be seen here, and are a Scheduled Ancient Monument.