The Silbury Hill of East Anglia?

Thetford Castle Hill – NHER 5747

No, despite its appearance, Thetford Castle Hill is not another Silbury Hill (a huge mound in Wiltshire dated to the Neolithic period) — it is clearly a medieval motte. From the bottom of its inner ditch, to the top of the mound, it is reputed to be one of the tallest man-made mounds in England at a height of over 80 feet. Local myths and legends surround it — secret tunnels, buried treasures etc. The Bailey of the castle is enclosed by huge earthwork ramparts to the north and to the east. These ramparts appear to have been constructed by reinforcing enclosure banks that were already standing before the 11th century, and are believed to have belonged to an Iron Age enclosure.

Modern synthesis within the county database (NHER 5747) heavily underscores this layered reuse of the topography. The colossal Norman motte — largely considered to have been commissioned by William de Warenne, 1st Earl of Surrey, shortly after the Conquest — was deliberately inserted into the pre-existing bivallate defences of the Iron Age fort. By utilising the massive prehistoric earthworks already commanding the confluence of the River Thet and the Little Ouse, the Norman builders saved immense labour while superimposing their own military monument directly onto an ancient regional locus of power.