One of the largest causewayed enclosures yet discovered in Britain. It has an area of about 8 ha, and its outer ditch has a diameter of approximately 360 m. The boundary earthworks comprise three roughly concentric rings of interrupted ditches which originally had internal banks. It is notable that the ditches do not lie around the top of the hill, but rather run down the northern slope. Windmill Hill was excavated on several occasions, most notably between 1925 and 1939 by Alexander Keiller, in 1957–8 by Isobel Smith, and in 1988 by Alasdair Whittle.
The earliest occupation dates to about 3800 bc, when there was a small unenclosed site in a woodland clearing. It was probably about 3500 bc that the enclosure was built, all three ditch circuits being open at broadly the same time. Within the enclosure ditches there is a considerable quantity of domestic rubbish (pottery, bones, flintworking waste, etc.). There was clear evidence of recutting in the ditches, suggesting that they were periodically cleared out. There were also a number of burials (mostly infants) on the ditch floors, and human bones were found scattered throughout the ditch fills. The enclosure ditches were not redug after about 2500 bc, although debris from later occupation, presumably from a settlement in the vicinity, found its way into the upper silting of the ditches. The purpose of the enclosure has been much debated. Isobel Smith suggested that it was a seasonal rallying point for dispersed farming communities who gathered there for festivals and trade, a view that is widely supported.
A. Whittle, J. Pollard, and C. Grigson, 1999, The harmony of symbols: the Windmill Hill causewayed enclosure. Oxford: Oxbow Books