Located in Castle Park is the Norman Castle mound, also known as Castle Hill which is the tallest medieval earthwork in Britain. The Castle that used to be situated at the top of the mound was one of many built by the Normans to establish control over the Kingdom.
The coordinates will take you to an information panel where you can obtain the numbers needed to find the cache which is at N52 24.ABC E000 45.DEF
There was a revolt against the Crown in 10A5
The religious houses fell into ruins in the 15B0s
King Stephen reigned 113C-54
How many different groups of people built these earthworks? = D
Thetford Spa was established in 18E9
The sum of the digits in the year King Swein's army burned the town = F
The cache is hidden not far away. You are looking for a bison tube.