It has stood firm for almost two millennia against the worst the weather can throw at it.

Whereas marauding invaders counted among its earliest enemies now it is more at risk from souvenir hunters prising a keepsake flint from its monumental walls.

And this week one of the guardians of Burgh Castle’s Roman Fort who mounts regular patrols moved to reassure visitors it was not falling over.

Despite reports that its precarious tilt had worsened due to heavy rain John Russell, who lives nearby, said surveys revealed it hadn’t moved even a millimetre in years.

“Someone raised the alarm that the south facing wall was leaning a bit more than it usually was but the section that is leaning is underpinned to rule out any problems with the soil.

“We have it inspected every year and measure it for lean. Following the last inspection in June last year there was absolutely no movement. It is probably just an optical illusion.

“At present the whole thing is balanced properly the only thing is that if water gets into the walls and opens up cracks you get mortar flaking off. The other threat is people taking flint out of the walls for souvenirs.

“We had similar reports to the parish council last year.”

At one point in the 11th century the Normans, under William the Conqueror, made alterations which included breaching a wall and adding a wooden fort on the mound – causing the precarious looking tilt to one of the walls which is properly underpinned, despite appearances.

Two years ago vandals toppled part of a smaller wall at the fort in what was tagged an “appalling” attack.

Dr Peter Wade-Martins of Norfolk Archaeological Trust which owns the site said: “When we bought the site we did check with English Heritage at the time and established there was a massive concrete foundation under the very leaning bit.”

The fort, one of the best preserved in Britain, was part of a coastal defence system built to protect England from invasion in the third and fourth century.

In places the walls are reckoned to be up to eight feet wide.