Cadbury Castle is a Bronze and Iron Age hillfort in the parish of South Cadbury in the English county of Somerset. It is a scheduled monument and has been associated with King Arthur's legendary court at Camelot.

The hillfort is formed by a 7.28 hectares (18.0 acres) plateau surrounded by ramparts on the surrounding slopes of the limestone Cadbury Hill. The site has been excavated in the late 19th and early 20th century by James Bennett and Harold St George Gray. More recent examination of the site was conducted in the 1960s by Leslie Alcock and since 1992 by the South Cadbury Environs Project. These have revealed artifacts from human occupation and use from the Neolithic through the Bronze and Iron Ages. The site was reused by the Roman forces and again from c. 470 until some time after 580. In the 11th century, it temporarily housed a Saxon mint. Evidence of various buildings at the site has been unearthed, including a "Great Hall", round and rectangular house foundations, metalworking, and a possible sequence of small rectangular temples or shrines.

The suffix -bury (from byrig, an Anglo-Saxon word meaning "fort" or "town") is frequently, but not exclusively, used to refer to hillforts. The first part of the name may come from the River Cam, as with the nearby villages of West Camel and Queen Camel. Other scholars suggest a derivation from some figure named "Cador" or "Cada". It is one of three sites in Somerset to include the Cadbury name, the others being Cadbury Camp, near Tickenham, and Cadbury Hill, which is also known as Cadbury-Congresbury to distinguish it from the other sites.

Hillforts developed in the Late Bronze and Early Iron ages, around the start of the 1st millennium BC. The reason for their emergence in prehistoric Britain and their purpose has been a subject of debate. It has been argued that they could have been military sites constructed in response to invasion from continental Europe, sites built by invaders, or a military reaction to social tensions caused by an increasing population and consequent pressure on agriculture. The dominant view since the 1960s has been that the increasing use of iron led to social changes in Britain. Deposits of iron ore were located in different places from the tin and copper ore necessary to make bronze and, as a result, trading patterns shifted. The old elites lost their economic and social status and power passed into the hands of a new group of people.

Archaeologist Barry Cunliffe believes that population increase still played a role and has stated that

the forts provided defensive possibilities for the community at those times when the stress of an increasing population burst out into open warfare. But I would not see them as having been built because there was a state of war. They would be functional as defensive strongholds when there were tensions and undoubtedly some of them were attacked and destroyed, but this was not the only, or even the most significant, factor in their construction.

Cadbury Castle is located 5 miles (8.0 km) north east of Yeovil. It stands on the summit of Cadbury Hill, a limestone hill situated on the southern edge of the Somerset Levels, with flat lowland to the north. The summit is 153 m (500 ft) above sea-level on lias stone.

The hill is surrounded by terraced earthwork banks and ditches and a stand of trees. On the north west and south sides there are four ramparts, with two remaining on the east. The summit plateau covers 7.28 hectares (18.0 acres), and is surrounded by the inner bank which is 1.2 kilometres (0.75 mi) long. The hillfort is overlooked by Sigwells, a rural plateau rich in archaeological remains. Due to scrub and tree growth the site has been added to the Heritage at Risk Register. Further information on the dig and hillfort can be found a short walk down the road on Somerset Heritage Panels which have been erected in the local pub The Camelot Inn.