The fort was named Anderida by the Romans and was built on what was than an uninhabited peninsula of land rising above the coastal marshes. The sea washed over what is now Pevensey Marshes surrounding Anderida on three sides, so giving a safe and sheltered landing point. This marshy inlet of the sea, extending inland as far a Hailsham, was studded with small areas of high land which remained as islands at high tide so giving the place-names of Rickney, Horse Eye, North Eye and Pevensey. All derived from the Old English word 'eye' meaning island.
When the Roman legions left Britannia in 408AD the native Britons (sinced Romanized) attempted to defend their island from attack. They were relatively successful until in 449AD their High King - a shrewd politician but inept general called Vortigern - payed these same enemies to help him as mercenaries and attack his enemies in the north - namely the Picts. The Jutes (led by King Hengist) were successful and were granted the island of Thanet in Kent for their troubles. However this plan backfired and the Jutes soon revolted and within ten years had captured Londonium (modern day London) and thrown Britain into disarray.
Following the Jutish example the Saxons began invading Britain in earnest. In 491 a Saxon army led by King Aelle landed on the south coast west of Kent and besieged Anderida. After an heroic battle the British defences were overrun and the entire garrison as well as scores of British refugees seeking shelter were massacred. The remaining Britons on the south coast either fled north in to the forests or by boat over the channel to found what is now called Britanny in France. King Aelle then declared that land to be the Kingdom of the South Saxons - later called Sussex - amd the old Roman fort of Anderida was burned and left derelect for 600 years.
For a while the ruined castle was known by the Saxons as Andredceaster and the enormous forest of southern England which stretched 120 miles from Anderida to Dorset was named Andredsweald or the Forest of Andred.
In time the Saxons eager to forget Britains Roman history renamed the ruined island Pefele (which means the Island of Pefe) which over the years became corrupted into the modern spelling of Pevensey. While England unified the fort of Anderida at Pevensey remained abandoned and derelict until in 1042 an Anglo-Saxon (English) noble (Harold Godwinson, Earl of Wessex - later King Harold II of England) established a strong point here, improving fortifications by digging ditches within the walls of the Roman Fort. The English army stayed at the fort during the summer of 1066 before abandoning it to meet the invading Norwegians further north. When the Duke William the Bastard of Normandy invaded Sussex in September 1066 there were no defences at Pevensey or anywhere else on the south coast.
In 1066 at the ensuing Battle of Hastings on Senlac Hill, Duke William defeated the combined English armies led by King Harold II. In late 1066 the castle at Pevensey was occupied by the Normans. Much of the Roman fort remaining on the castle site is due largely to the work of Robert of Mortain (half brother to William the Conqueror), who was granted Pevensey Castle shortly after the Norman Conquest. De Mortain used the existing fort as the base for building his castle, carrying out only minor repairs to the walls forming the outer bailey, and building a new inner bailey at the eastern end.
A new gateway replaced the original main entrance to the southwest, and the east gateway was repaired. Other alterations made were mainly additions and improvement to existing structures within the original fort. An irregular, rectangular-shaped enclosure was created using part of the Roman wall and two bastions on the southeastern side. Shortly after the inner bailey was created, the rectangular stone keep was erected, incorporating part of the east curtain wall and a Roman bastion. Some time later, three more bastions facing the inner bailey were added to the keep.
The castle was besieged by William Rufus in 1087 and during a period of civil war by the forces loyal to King Stephen (1135-1141). Simon de Montfort on his way back fromt taking Lewes besieged the castle in 1264 and in 1399 it was again attacked during the War of the Roses.
During later times the ancient castle nearly didn't make it. Queen Elizabeth I ordered the castle to be demolished but this was ignored and during the period of interegnum under Oliver Cromwell efforts were again made to destroy it but luckily only a few stones were removed. As late as 1942 small additions were made to the castle for the defence of Britain when it became a look-out over the channel for invading German warplanes during World War II.