Once taken to mean Caesar's fort (after Julius Caesar) or Cissa's fort (after South Saxon king Ælle's son, Cissa), both theories of Cissbury's meaning have been discounted. In the early eleventh century in the reign of Ethelred the Unready, Cissbury was known as 'Sith(m)esteburh', which is taken to mean the 'last or the latest fort'. This may mean that Cissbury was the last fort to be refortified, after another nearby fort such as that at Burpham. It seems that the name Cissbury was altered to accommodate the legend that the fort was linked to Cissa the South Saxon.
Cissbury Ring is a hill fort on the South Downs, in the borough of Worthing, and about 4 miles (6.4 km) from its town centre, in the English county of West Sussex. It is the largest hill fort in Sussex, the second largest in England and one of the largest in Britain and Europe overall, covering some 60 acres (24 hectares). Several Bronze Age barrows have been found just outside Cissbury Ring. The earthworks that form the fortifications were built around the beginning of the Middle Iron-Age possibly around 250 BC but abandoned in the period 50 BC - 50 AD.
The ditches and banks are the remains of a defensive wall that enclosed 65 acres (260,000 m2) of land; the inner band of the wall is over a mile around. The ditches are said to be as deep as three metres and were filled with loosened chalk and covered with timber palisade. The 600-foot (184m) hill is open to the public but the climb is said to be "not for the faint hearted." From the top, one is able to see to the west Selsey, Chichester Cathedral, the Spinnaker Tower and the Isle of Wight. To the east, one is able to see Brighton, the Seven Sisters and Beachy Head. Cissbury Ring is the highest point in the borough of Worthing.
Romano-British farmers settled within the ramparts of the hill fort. It has been suggested that a medieval mint which produced coinage existed at Cissbury around the eleventh century. Coins have been found across Sussex from Chichester to Lewes bearing the name Sithe, Sithsteb and Sithmes, taken to mean the former name of Cissbury in use at this time of 'Sith(m)esteburh'. The coins found are from the reigns of Ethelred the Unready and Cnut. Although it is quite possible that a mint existed at Cissbury, no trace of it has yet been found.
During World War II, Cissbury Ring was used as a camp for the 2nd Argyll and Sutherland Highlanders in preparation for the Normandy landings. At this time much of the fort within the ramparts was ploughed to provide food. While on manoeuvres, tanks destroyed the dew pond at the north side of the fort. A gap was made in the ramparts to accommodate a 100 lb (45 kg) gun which was used to fire at ships in the English Channel and an anti-aircraft gun was sited by the gap. Cissbury Ring is owned by the National Trust and is a legally-protected Scheduled Ancient Monument. It also lies within the area of the proposed South Downs National Park. Artefacts from Cissbury Ring can be found at Worthing Museum and Art Gallery, the Museum of Sussex Archaeology, Lewes and the Ashmolean Museum, Oxford. Silver pennies from the reign of Ethelred II (978-1016) can be found at the British Museum.
Cissbury Ring was said to have been formed when the Devil tried to dig a hole in the South Downs, to allow the sea to flood the Sussex Weald and all its churches. As the devil dug out Devil's Dyke, clods of earth fell to the ground forming Cissbury, Chanctonbury, Rackham Hill and Mount Caburn.
“At Offington, near Worthing, an old seat of the Delawarrs, a blocked-up passage, which can only be approached from the cellars, is still believed to communicate with the encampment on Cisbury Hill, and to be full of buried treasures. Some years ago there was a story current of the then occupier of the house having offered half the money to be found there to anybody who would clear out the subterranean passage, and that several persons had begun digging, but had all been driven back by large snakes springing at them with open mouths and angry hisses.”
From 'Some West Sussex Superstitions Lingering in 1868' by Charlotte Latham, in 'The Folk-Lore Record' Vol. 1. (1878), p16.