Corfe Castle is a fortification standing above the village of the same name on the Isle of Purbeck peninsula in the English county of Dorset. Built by William the Conqueror, the castle dates to the 11th century and commands a gap in the Purbeck Hills on the route between Wareham and Swanage. The first phase was one of the earliest castles in England to be built at least partly using stone when the majority were built with earth and timber. Corfe Castle underwent major structural changes in the 12th and 13th centuries.

In 1572, Corfe Castle left the Crown's control when Elizabeth I sold it to Sir Christopher Hatton. Sir John Bankes bought the castle in 1635, and was the owner during the English Civil War. While Bankes was fighting in London and Oxford, his wife, Lady Mary Bankes, led the defence of the castle when it was twice besieged by Parliamentarian forces. The first siege, in 1643, was unsuccessful, but by 1645 Corfe was one of the last remaining royalist strongholds in southern England and fell to a siege ending in an assault. In March that year Corfe Castle was slighted on Parliament's orders. Owned by the National Trust, the castle is open to the public and in 2018 received around 237,000 visitors. It is protected as a Grade I listed building and a Scheduled Monument.

Corfe Castle was built on a steep hill in a gap in a long line of chalk hills, created by two streams eroding the rock on either side. The name Corfe derives from the Old English ceorfan, meaning 'a cutting', referring to the gap. The construction of the medieval castle means that little is known about previous activity on the hill. We know from contemporary writing that Anglo-Saxon nobility treated it as a residence, such as Queen Ælfthryth, wife of Edgar, and there are postholes belonging to a Saxon hall on the site. This hall may be where the boy-king Edward the Martyr was assassinated in 978; contemporaries tell us that he went to the castle at Corfe to visit Ælfthryth and his brother.

A castle was founded at Corfe on England's south coast soon after the Norman conquest of England in 1066. The royal forest of Purbeck, where William the Conqueror enjoyed hunting, was established in the area. Between 1066 and 1087, William established 36 such castles in England. The castle stood 21 m (69 ft) tall atop a 55 m (180 ft) high hill. Sitting as it does on a hill top, Corfe Castle is one of the classic images of a medieval castle. However, despite popular imagination, occupying the highest point in the landscape was not the typical position of a medieval castle. In England, a minority are located on hilltops, but most are in valleys; many were near important transport routes such as river crossings.

Unusually for castles built in the 11th century, Corfe was partially constructed from stone indicating it was of particularly high status. A stone wall was built around the hill top, creating an inner ward or enclosure. There were two further enclosures: one to the west, and one that extended south (the outer bailey); in contrast to the inner bailey, these were surrounded by palisades made from timber. At the time, the vast majority of castles in England were built using earth and timber, and it was not until the 12th century that many began to be rebuilt in stone. The Domesday Book of 1086 records one castle in Dorset; the entry, which reads "Of the manor of Kingston the King has one hide on which he built Wareham castle", is thought to refer to Corfe rather than the timber castle at Wareham. There are 48 castles directly mentioned in the Domesday Book, although not all those in existence at the time were recorded. Assuming that Corfe is the castle in question, it is one of four the Domesday Book attributes to William the Conqueror; the survey explicitly mentions seven people as having built castles, of whom William was the most prolific.

In the early 12th century, Henry I began the construction of a stone keep at Corfe. Progressing at a rate of 3 to 4 metres (10 to 13 ft) per year for the best part of a decade, the work began in around 1096 or 1097 and was complete by 1105. The chalk of the hill Corfe Castle was built on was an unsuitable building material, and instead Purbeck limestone quarried a few miles away was used. By the reign of King Stephen (1135–1154) Corfe Castle was already a strong fortress with a keep and inner enclosure, both built in stone. In 1139, during the civil war of Stephen's reign, Corfe withstood a siege by the King. It is thought that he built a siege castle to facilitate the siege and that a series of earthworks about 290 metres (320 yd) south-southwest of Corfe Castle mark the site of the fortification.

During the reign of Henry II Corfe Castle was probably not significantly changed, and records from Richard's reign indicate maintenance rather than significant new building work. In contrast, extensive construction of other towers, halls and walls occurred during the reigns of John and Henry III, both of whom kept Eleanor, rightful Duchess of Brittany who posed a potential threat to their crowns, in confinement at Corfe until 1222. John also kept Scottish hostages Margaret of Scotland and Isabella of Scotland there. In 1203, Savari de Mauléon was also imprisoned, but reputedly escaped by getting his jailers drunk and then overpowering them. It was probably during John's reign that the Gloriette in the inner bailey was built. The Pipe Rolls, records of royal expenditure, show that between 1201 and 1204 over £750 was spent at the castle, probably on rebuilding the defences of the west bailey with £275 spent on constructing the Gloriette. The Royal Commission on the Historical Monuments of England noted the link between periods of unrest and building at Corfe. In the early years of his reign, John lost Normandy to the French, and further building work at Corfe coincided with the political disturbances later in his reign. At least £500 was spent between 1212 and 1214 and may have been focused on the defences of the outer bailey. R. Allen Brown noted that in John's reign "it would seem that though a fortress of the first order might cost more than £7,000, a medium castle of reasonable strength might be built for less than £2,000". The Pipe Rolls show that John spent over £17,000 on 95 castles during his reign spread; he spent over £500 at nine of them, of which Corfe was one. Additional records show that John spent over £1,400 at Corfe Castle.

One of the secondary roles of castles was to act as a storage facility, as demonstrated by Corfe Castle; in 1224 Henry III sent to Corfe for 15,000 crossbow bolts to be used in the siege of Bedford Castle. Following John's work, Henry III also spent over £1,000 on Corfe Castle, in particular the years 1235 and 1236 saw £362 spent on the keep. While construction was under-way, a camp to accommodate the workers was established outside the castle. Over time, this grew into a settlement in its own right and in 1247 was granted a market and fair by royal permission. It was Henry III who ordered in 1244 that Corfe's keep should be whitewashed. Four years previously, he also ordered that the keep of the Tower of London should be whitewashed, and it therefore became known as the White Tower.