Thetford Castle Hill is an enormous mound, partly sunk into a deep surrounding ditch, and two sets of ramparts. The hill, or mound, is a Norman motte earthwork. It is the highest Norman motte in England, though no trace remains of the castle which once surmounted it.
There were two motte and bailey castles, made of timber, on the site. The first, Red Castle, was erected during the end of the 11th century (circa 1067), following the Norman invasion. The second, Castle Mound, was erected during the time of the civil war (1135 -1154) between the supporters of King Stephen and the Empress Matilda. The wooden buildings were destroyed in 1174. The earth ramparts of the castle defences are believed to have been built on top of much earlier defensive ramparts, belonging to an Iron Age hillfort, dated to somewhere around 400 BC to 100 BC.
The castle guarded the Little Ouse and controlled important crossings, at what is now Nuns Bridges, of the rivers Thet and Ouse. The medieval market was created close to the castle at Old Market Street.
Today the site is part of a large green area known as Castle Park, a public park. Children’s play areas have been laid out close to the ramparts. The mound (motte) is open to the public year round. (Wikipedia)