According to the Anglo-Saxon Chronicle Ælle accompanied by his three sons Cymen, Wlenca and Cissa landed in three ships at a place called Cymensora in 465AD and fought the local inhabitants.
The Anglo-Saxon Chronicle was commissioned by King Alfred, towards the end of the 9th century, when several hundred years had passed since the landing at Cymensora. The chronicle continued till 1154 AD, well after the Norman conquest.
The Anglo-Saxons would duplicate important documents and distribute them around the kingdom, enabling the king or his nobles to access a copy wherever they were in the domain. The authors, of the chronicle, tended to include information from the area they were in, so there are variances between documents depending on the editor and part of the country they came from, also we do not know how accurate a transcription they made from the earlier sources. However the placenames are thought to be genuine and that of Cymensora is believed to have been in the Selsey area.
Saint Wilfrid was given land in the Selsey area by King Caedwalla in the 8th century, and documents, known as the Saxon Charters, confirming the grant were drawn up, but yet again they were not produced in the time of Wilfrid, but a couple of centuries later. Despite the doubt about their authenticity, what makes them interesting, to us, is that recognisable place names are used for the boundaries. The placenames are again are thought to be accurate, as the the charters were legal documents drawn up to support the Bishop of Selsey's claim on Pagham Harbour against the Archbishop of Canterbury, who owned much of the land in the Pagham area. Pagham Harbour at the time was a wealthy trading area. Most of Pagham Harbour to this day lay within the parish bounds of Selsey. One of the placenames in the document is Cummenshore and this is thought to be an alternative spelling for Cymensora (where Ælle landed). The historian Susan Kelly discusses this in her book Anglo-Saxon Charters VI - "Charters of Selsey", ed. SE Kelly, British Academy, pub. OUP, 1998 - page 12.
The relevant part of the boundary clause states that "from the entrance of the harbour which is called in English Wyderinges [Pagham Harbour]" the boundary ran along the coast to "Cummenshore".
Although most academics agree that Cummenshore and Cymenesora are the same place they are not so sure about the existance of Ælle or his sons. The Venerable Bede describes Ælle as the first King of all the Saxons but provides us with no more information than that. His writings were in the 8th century so still a couple of hundred years after Ælle was supposed to have arrived and Bede does not provide details of a landing place.
And so Duke Ælla, with his sons and a fleet that was well equipped with fighting men, landed in Britain at Cymenes ore When the Saxons disembarked, however, the Britons raised the alarm and a great number rushed from the .surrounding district and immediately gave battle. But the Saxons who were much taller and stronger , received their disorganised attacks with disdain. For coming in small groups at intervals, they were slaughtered by the Saxons cohesive force, and as each wave returned in shock, they heard the unexpectedly bad news. So the Britons were driven to the nearest forest, which is called Andredesleige [the Weald]
The Saxons occupied the Sussex coast, seizing control of more and more of the region up to the ninth year after their arrival But then, when they had more audaciously taken. over a distant district, the British kings and rulers met at Mecredesburne and fought against Ælla, and his sons. The issue was undecided. Both armies suffered grave injuries and losses, and each vowing friendship with the other they returned to their own lands. Therefore Ælla sent to his compatriots seeking aid. The year in which Ælla came to England was almost the thirtieth after the coming of the English.
Henry of Huntingdon, Historia Anglorum, c.1135
So where is the location for Cymensora? Other than some gold ring fragments, possibly dating from the 6th or 7th century, and some speculation of an early Saxon graveyard near Pagham church ( after a burial urn was found in the area) there has not been any archaelogical evidence to show a Saxon settlement in Selsey at the time of Ælle. There has been 1500 years of erosion and the shoreline would have been 2 - 3 km more seaward at the time of Ælle. Thus any early Saxon settlement could well be under the sea.
However we still have the Saxon Charters and they do give a description for the location of Cymensora. Historians have suggested that the Owers rocks have their name derived from ora as in Cymensora. Local historians Edward Heron-Allen and Frances Mee cite Keynor in the Parish of Sidlesham as a possible site. Maybe the name Keynor being a corruption of Cymensora, the local pronunciation for Keynor is Cy-nor. The amateur marine archaeologist Hume -Wallace, dived extensively off the Selsey coast and suggested that the Mixon rocks are a possible place, they were not inundated by the sea till the 10th century.
The speculation will continue on Ælle's existance and the location of Cymensora until someone discovers some supporting archaeology. Remember that the Mary Rose was discovered due the persistance of one man (Alexander McKee) searching over many years for the wreck. This is unlikely to happen in regard to the search for Cymensora.