According to the Chronicle, Aelle arrived in Sussex in 477 with three ships and his three sons - Cymen, Wlenking, and Cissa - and landed at a place called Cymenshore. He killed the local British defenders, drove the survivors into the great forest of Andred (Andredsleag), and over the following years fought two more battles to consolidate his hold. The second of these was Mercredesburne in 485. The third was the besieging of Pevensey in 491, after which the Chronicle bluntly states the Saxons "slew all the inhabitants; there was not even one Briton left there." Aelle is named in the Chronicle as the first Bretwalda - overlord - of the southern Anglo-Saxon kingdoms, and the founder of what became the Kingdom of Sussex ("South Saxons"). His son Cissa is said to have given his name to Chichester (Cissa's ceaster, Cissa's Roman town). The Kingdom of Sussex itself would last until the 9th century, when it was absorbed into the West Saxon kingdom of Wessex.

The place-name itself has been the strongest clue in the long search for the battlefield. The historian and archaeologist Martin Welch read Mercredesburne as something like "river of the frontier agreed by treaty" - in which case the name itself might commemorate a peace settlement between Aelle and the local British, fixing the river concerned as a boundary. This would fit a story preserved by the 9th-century Welsh chronicler Nennius, who describes how the British leader Vortigern was lured into treating with the Saxon Hengest. According to Nennius, three hundred British nobles met Hengest's men for the negotiation; the Saxons murdered them all after getting them drunk, and forced Vortigern to cede Sussex by treaty. Whether any of this happened in any literal sense is anyone's guess, but the etymology of Mercredesburne does at least hint that the battle marked a border.

The villages of Ashburnham and Penhurst in East Sussex have long held a tradition that an ancient earthwork called Town Creep, in Creep Wood between the two villages, was the site of Mercredesburne. Oral tradition surviving into the late 19th century described Town Creep as the site of a town besieged and destroyed by the Saxons. In 1896 members of the Sussex Archaeological Society went and investigated; they concluded the earthwork was a plausible location, suggested that the modern name might preserve a corruption of "Mercrede," and that the "burn" element might refer to the Ashburn stream running beneath the earthwork. A separate theory places "Mearcredesburnan stede" at modern Binstead, on a reading of the Old English meaning "the sea landing stage at the watercress stream." Both theories are plausible. Neither is provable. The site, like so much of fifth-century England, is gone behind a curtain of centuries.

The Anglo-Saxon Chronicle is one of the great early medieval European historical sources, but it was compiled in Alfred's Wessex in the late 9th century - four hundred years after Aelle's reign. The early Anglo-Saxons were not literate in the Latin and Greek sense; they had runes, but their culture was emphatically oral. They did not really begin to write down legal and historical events until they were Christianised, which for the South Saxons did not happen until the late 7th century, two centuries after Aelle's death. The chroniclers reaching back to the 5th century were drawing on the kind of source you might expect: heroic poetry, royal genealogies designed to flatter living kings, fragmentary memories pieced together long after the events. The 14th-century chronicler Roger of Wendover even names the British leader at Mercredesburne as Aurelius Ambrosius - the same Romano-British figure who appears in legend as the uncle of King Arthur. That identification is romantic embroidery rather than evidence.

What remains, then, is something more like a story than a known event. A Saxon warlord landed on the Sussex coast in the late 5th century with his sons and a small fleet. He pushed inland, fought the Romano-British populations who still held what remained of post-Roman southern England, and eventually carved out a kingdom that would last almost four centuries. Somewhere in the middle of that conquest, in 485, he fought a battle by a river. The Britons may have been led by a man whose name we have lost. The defeated may have been buried in earthworks that still exist as low ridges in Creep Wood, where rabbits dig and beech leaves accumulate on the ground in autumn. Henry of Huntingdon, working in the 12th century, claimed Aelle died in 515 and was succeeded by his son Cissa. After Cissa, the South Saxon kings disappear from the record for over a century, suggesting that the early Sussex monarchy was, like the battles that founded it, a small affair conducted mostly in the shadow of forests that have since been cleared.