The tree-covered heights of the Iron Age Hillfort on Sinodun (or Castle) Hill are a well-known beauty spot known variously as Wittenham Clumps, the Berkshire Bubs or Mother Dunch's Buttocks! The name Sinodun is pure Celtic. Seno-Dunum means 'Old Fort'. This may indicate it was abandoned quite some time before the Romans arrived in Berkshire or, more likely, that it was so-named in the post-Roman period, before the Saxons arrived.

Excavations have shown that a late Bronze Age sub-circular enclosure surrounded the first hectare of settlement on the site. It had a 2.5m deep U-shaped ditch and associated bank, possibly with a palisade on top. The early Iron Age saw the building of the banks and 7.5m deep V-shaped ditches forming the hillfort of today. There was a rampart with both timber palisades and revetment. No round houses have been discovered so far, but the inhabitants, who were largely resident in the middle Iron Age, are known to have eaten fish from Thames and wild boar from the woods, whilst farming some cattle, but mostly sheep. Barley and some wheat was grown in the surrounding fields and stored here in huge pits. Other activities in evidence include sewing (with bone needles) and spinning (with spindle whorls). Unusually for the Iron Age, a number of burials were discovered - one in a grave, the others in pits. The most significant consisted of a large male buried in the foetal position at the bottom of a pit, a charred grain deposit at his feet and joints of meat under his arm. After a covering of earth, the better part of a dismembered female was placed on top of him. The final covering included the body of a sheep. All were deposited at one go and it has been suggested that the cut marks on the female bones indicate she was a human sacrifice, as described by Roman writers. There appears to have been no occupation in the late Iron Age, but Roman rubbish dumping corresponds with the establishment of one, if not two, Roman buildings in large enclosures, to the south-west. Rectangular features, upwards of 3m wide, also date from this period. They appear to be water storage tanks associated with nearby springs. No evidence of Saxon occupation has been found, but King Offa of Mercia apparently built some sort of look-out post at Sinodun, after defeating the West Saxons at the Battle of Benson in AD 772.

The Clumps are the home of the Victorian 'Poem Tree' and an old oak on the Wittenham estate was said to have been the one under which Matthew Prior wrote another poem, his famous 'Henry & Emma'. Also at the clumps is the 'Money-Pit' where a vast treasure, guarded by a large black raven, is said to be buried. The clump of trees on Round (or Harp) Hill, adjoining Sinodun, are sometimes called the Cuckoo Pen. The locals believed that if you could trap a cuckoo within its branches, summer would never end.

The name of the village could derive from Vedonium, Brito-Latin for 'Wet Town', referring to the flooding river rather than inclement weather. A Saxon phrase meaning 'Witta's Water Meadow' is the more usual interpretation. The place eventually became known as Abbot's Wittenham, for it was owned by Abingdon Abbey from around 1048. Their grange, adjoining the church, appears to have been quite important to the monks. It supplied the abbey with fowl, eggs, lentils and wood, while the fishery was appropriated for the especial use of the kitchener. The manor house which replaced it was probably built by William Dunch of London. He purchased the estate in 1552, after it had been seized from the Abbey during the Dissolution. Several of his important family were Sheriffs of Berkshire and they lived at Wittenham for many years. There is an interesting renaissance effigial monument in the church to his grandson and knighted namesake. His wife, Mary Cromwell, rests beside him. She was the Protector Oliver Cromwell's aunt. Old illustrations show the monument was once more elaborate than it is today, but, when the church was rebuilt in 1862, it was moved from its original position in the old south aisle and had to be made to fit its present resting-place. Edmund Dunch (III), naturally enough, supported his cousin, Cromwell, during the Civil War. John Hampden was also a cousin but, when Richard Cromwell failed to keep the country together, he assisted in the restoration of Charles II. Despite this, he was still stripped of his title, Lord Burnell of East Wittenham, granted him in honour of his wife. The last of the line, Edmund Dunch (IV), died in 1719 when the manor came to three heiresses and subsequent absentee owners. The old manor house was in a state of great decay by 1784 and demolished soon afterward.