The Romans built their stronghold 2,000 years ago, to fortify the coastline.
Now the ancient fort has been closed to the public, because a man has been grazing his horse on it.
The National Trust which owns Branodunum, near Brancaster, said it had temporarily closed the site for public safety reasons due to "an ongoing situation with fly-grazing".
The problem centred on a single horse, which was moved on by its owner last Friday.
The animal has now been removed from the site, beside the main A149 coast road, but gates remain locked.
A National Trust spokesman said: "We are working to resolve the issue of fly grazing on the field at Branodunum.
"We have been in contact with the horse’s owner and have been exploring a number of options as part of a multi-agency response, to find a suitable home for the horse.
"The owner has now removed his horse from the site, which remains temporarily closed until we’re confident that the matter has been resolved.”
Branodunum is mainly used by villagers to walk their dogs and admire the view across the marshes.
It once sprawled across six acres with a ditch, earthen ramparts and 10ft-thick walls to protect its garrison.
It was one of eleven forts built around the east and south coasts of England by the Romans in the 3rd century.
They remained in use for more than a century until the Romans withdrew in 410AD because its army was needed to defend its homeland.
Branodunum would once have stretched down to the sea but the shoreline has receded since Roman times.
Today little of Branodunum's earthworks remain, while the remains of a civilian settlement to the west lies under a 1970s housing estate named after it.
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