Tests show Skipsea Castle is Britain's largest Iron Age mound
A 40ft (12m) mound in East Yorkshire originally thought to have been built by the Normans is actually 2,500 years old, archaeologists have found.
The finding makes Skipsea Castle the largest Iron Age mound to be discovered in Britain - and one of the largest in Europe. It is believed the motte and bailey castle was built on top of a mound that was already at the site, researchers said.
University of Reading archaeologists said Skipsea Castle is more similar to Silbury Hill in Wiltshire than previously thought.
Silbury Hill is the largest man-made mound in Europe and is thought to have been completed in around 2400 BC, making it around 2,000 years older than Skipsea Castle.
Before the discovery about Skipsea Castle, dubbed the "Silbury Hill of the North" , experts knew of only smaller mounds from the Iron Age. The closest mound of a similar size is in Germany.
Dr Jim Leary, the University of Reading archaeologist who led the excavation, said: "To say that the discovery of an Iron Age monument hiding in plain sight was surprising is an understatement.
"Conventional wisdom has suggested that castle mottes were brought to England by the Normans, following the conquest that began in October 1066, exactly 950 years ago.
"Castle mottes exist up and down the country, but their huge size means they are rarely excavated and as a result much of what we previously thought we knew about their date was based on scant documentary evidence and guesswork."
Archaeologists extracted earth from the heart of the mound and used radiocarbon dating to measure how old it was.
Dr Leary added: "We are able, for the first time, to reveal the date of construction, sequence of development, and environmental context of these wonderful monuments, providing insights never previously thought possible."
He told the BBC the "key question" is to work out the purpose of the mound. Similar mounds across Europe were used for burials.