Historians believe the castle may have had the top storey removed
Colchester Castle, one of the most outstanding examples of Norman heritage is to be the subject of a BBC 2 series as part of the BBC's Norman season.
Other fine examples include Hedingham and Copford churches, Mountfitchet Castle and St Botolph's Priory.
Presenter Professor Robert Bartlett told BBC Essex he believed the reason the Normans were so successful was largely due to their castles.
"The biggest of them all and the most dramatic is Colchester," he said.
He added: "Colchester must have been one of the biggest castles in Europe at that time."
The Normans used some of the old Roman brick that was lying around the town to build the castle on the site of the ruined Roman temple.
"One of the things the Normans did when they conquered England was to draw up this amazing survey, the
said Professor Bartlett.
Today there are two surviving copies, one of which has more detailed information than the other.
"Essex, fortunately, is in that more detailed survey," said Professor Bartlett.
"So you can go to the Domesday book, there's a translation into English now, and find a detailed account of what your village in Essex was like in the year 1086 and that's not true of any other part of Europe," he added.
You can discover more about Essex's unique Norman history with our