St Ann’s Hill is a prominent gravel-capped knoll rising out of the Thames Valley. Traces of a defensive earthwork were formerly visible on the hill, which was once known as Eldebury or Oldbury Hill. Today, a long line of scarp with a ditch at its foot extends along the western crest of the hill. The southern end is terminated above the Dingle and the northern end finishes abruptly with the hillside continuing beyond in a way to suggest that there were never any further artificial works in that direction, as the steep slopes become naturally defensive in themselves at this point. The summit of the hill is approximately level and assuming the work on the west to have swung round on the same curve to the south-east apex, some 12 acres would have been enclosed.
The site, a Scheduled Monument, was excavated in 1990 to provide information for management of the monument. Evidence was found for use and occupation over a long period of time, for example, Mesolithic worked flints including cores were found showing the site was in use between 10,000 and 6,000 years ago. The postulated defences were sectioned and the site was confirmed as an Iron Age univallate hillfort and a full detailed survey was carried out by English Heritage. A small trench in the interior identified 53 prehistoric features, mostly of early to middle Iron Age date, with post-holes indicating three or more building phases.