Historians are divided over whether or not Ælle really existed; however archaeological evidence supports the view that a short-lived expansion of South Saxon authority as far as the Midlands may have taken place in the 5th century.

[22] In the early mediaeval period, the rivers of Sussex may have acted locally as a major unifier, linking coastal, estuary and riverside communities and providing people in these areas with a sense of identity.

Eventually traders gravitated to churches, founding villages, and in some cases market towns such as Ditchling, Shermanbury, Thakeham, Ashurst and Shipley.

The Anglo-Saxon Chronicle goes on to describe Ælle's battle with the British in 485 near the bank of Mercredesburne, and his siege of the Saxon Shore fort at Andredadsceaster (modern day Pevensey) in 491 after which the inhabitants were massacred.

[43] The principal area of settlement in the 5th century has been identified as between the lower Ouse and Cuckmere rivers in East Sussex, based on the number of Anglo-Saxon cemeteries there.

[52] J. N. L. Myres posits that archaeological evidence, in the form of distinctive Saxon saucer brooches, suggests that Ælle's forces penetrated north as far as modern day Oxfordshire and Gloucestershire to the west.

[57] The abbey eventually became the seat of the South Saxon bishopric, where it remained until after the Norman Conquest, when it was moved to Chichester by decree of the Council of London of 1075.

[57][58] Shortly after the arrival of St Wilfrid, the kingdom was ravaged with "fierce slaughter and devastation" and Æðelwealh was slain by an exiled West Saxon prince Cædwalla.

At this time, a new South Saxon hegemony extending from the Isle of Wight into Kent could conceivably have seen Sussex re-emerge as a regional power but the revival of Wessex ended this possibility.

[60] According to Bede, the subjection reduced the kingdom of Sussex to "a worse state of slavery"; it also included placing the South Saxon clergy under the authority of Wessex through the bishops of Winchester.

[62] There is a theory that Watt may have been a sub-king who ruled over a tribe of people centred around modern day Hastings, known as the Haestingas and Nunna is described, in the Anglo-Saxon Chronicle, as the kinsman of Ine of Wessex who fought with him against Geraint, King of the Britons, in 710.

It is probable that about this time Offa annexed the kingdom of Sussex, as several persons, Osmund, Ælfwald and Oslac, who had previously used the royal title, now sign with that of dux.

According to the abbey's records, in which he was called princeps Australium Saxonum, Eadwinus nomine (Eadwine leader of the South Saxons), he bequeathed estates to them in his will, although the document itself has not survived.

In an early example of local government reform, the Anglo-Saxon ealdormanries were abolished by the Danish kings and replaced with a smaller number of larger earldoms.

[92] Alfred the Great almost certainly inaugurated the building of a series of burhs or forts to be garrisoned at the threat of danger by men drawn from the surrounding population.

The development of the burhs across the southern half of England suggests a considerable awareness of a repeated problem[92] The Burghal Hidage documents five such fortifications in Sussex — at Chichester, Burpham, Lewes, Hastings and Eorpeburnan.

In the reign of Æthelred the Unready, the threat of the Danes continued — in 994 and 1000 the Anglo Saxon Chronicle records burning, plundering and manslaughter on the coast of Sussex and neighbouring counties.

A new mint also seems to have existed on a temporary basis in the Iron Age hillfort at Cissbury, which may have been refortified as a refuge during the Danish invasions in the reign of Æthelred the Unready.

[96] The substantial sea-faring trade of Lewes is indicated by the payment of 20 shillings for munitions of war payable whenever Edward the Confessor's fleet put to sea.

[103] The fact that the Sussex coast appears to have been relatively densely settled for centuries implies that the land was being more competently farmed than was typical of the standard of the day.

The archaeologist Martin Biddle said that "the evidence we have for the residences and itineraries of English kings before the Norman conquest is all too thin" [107] and according to Frank Stenton "In the eleventh century the conception of a capital city had not yet taken a definite shape anywhere in the west.

[109] After the departure of the Romans, Noviomagus appears to have been largely abandoned with the earliest Saxon find, by archaeological excavation, being a small amount of mid-Saxon pottery dated around 8th-9th century.

[114][115] According to Martin Welch "After the Romans left there is no evidence for the reoccupation of Chichester till the 9th century", when it was rebuilt and fortified as part of a programme of defence, instigated by Alfred the Great, of Wessex, to protect against Danish raids.

Though in part due to the careful cultivation of conquered regions, the establishment of an enduring "Greater Wessex" stretching along the southern coast owed much to chance, early deaths, and perhaps, to the growing recognition of the need for unity in the face of an increasing Viking threat.

[126] The early hundreds often lacked the formality of later attempts of local government: frequently they met in the open, at a convenient central spot, perhaps marked by a tree, as at Easebourne.

[128] It is also recorded that an England-wide Royal Council (Witenagemot) took place in Sussex on 3 April 930,[129] when Æthelstan, the first king of the English, and his councillors gathered at Lyminster by the River Arun.

In 681 AD Saint Wilfrid, the exiled Bishop of York, landed at Selsey and is credited with evangelising the local population and founding the church in Sussex.

[139] In the late 7th or early 8th century, St. Cuthman, a shepherd who may have been born in Chidham and had been reduced to begging set out from his home with his disabled mother using a one-wheeled cart.

[146] Wilfrid's first act after he was given land at Selsey by King Æðelwealh was to build a monastery to free 250 male and female slaves from slavery who were tied to the estate.

Archaeological evidence shows that luxury food items were consumed in Sussex and exuberant architectural displays were constructed, such as a cellared tower excavated at Bishopstone.