It is thought that the tunnel was built as part of a Norman plan to undermine Exeter's walls and cause them to collapse. However, after an 18-day siege, it seems that the city surrendered before the Norman sappers implemented the final stage of their plan.
According to the medieval chronicler Orderic Vitalis, another Norman tunnel, built under the wall itself, caused part of the defences to collapse, thus forcing the citizens to surrender. After William took the city it appears that the tunnel was abandoned.
So far a 30ft length of the 3ft-high tunnel has been discovered by archaeologists from Exeter City Council's museum service.
It appears to have started below the city wall in the side of what was the dry moat which surrounded Anglo-Saxon Exeter. The Norman sappers, digging the start of the siege tunnel, would have worked under some sort of mobile protective structure for protection from arrows and other projectiles fired from the city wall.
Once under the wall, the tunnelers veered north-west 45 degrees so that they were burrowing directly underneath the city's East Gate.
The Norman siege of Exeter took place in 1068, 18 months after the Battle of Hastings. Most of Exeter's citizens were implacably hostile to the Normans, although some of the city's top merchants wanted to negotiate a compromise deal with the invaders to turn Exeter into an Italian-style city-state republic, nominally within a wider Norman empire.
William turned this down bluntly: 'It is not my custom to take subjects on such conditions,' he told a civic deputation.
The Normans then advanced towards Exeter and met a second deputation which negotiated a surrender and gave hostages as a guarantee of submission.
However, the city elite were overthrown by the populace in some sort of bloodless coup and could not deliver what they had promised to William.
William found the gates barred and the citizens defiant. One English fighter standing on top of the city wall offered the Normans the ultimate insult by dropping his trousers and loudly breaking wind.
But the Norman sappers saw to it that 18 days later Exeter surrendered - and William marched in triumph through the very gate that he had planned to destroy.
Exeter archaelogists have also discovered a 12th-century earthwork siege fort, built by King Stephen when he besieged Rougemont Castle in 1136. Excavations have revealed that the circular stronghold was 180ft across.