Workers repaving Cornmarket Street in Oxford discovered ancient silt that suggests the possible existence of a drawbridge in Norman times.
Contractors have fenced off part of the city's main shopping street to carrying out a £2m improvement scheme.
When new surface-water drains were being installed outside Bos- well's, contractors discovered soft grey silt instead of yellow gravel and alerted archaeologists.
Experts from Oxford Archaeological Unit, based in Osney Mead, have taken samples and are carrying out tests on the silt, which could date from the 12th century.
City council archaeologist Brian Durham an said the silt could indicate that a passage of water once flowed beneath a drawbridge in front of the city's North Gate.
He added: "There was a gate tower at Folly Bridge which had a drawbridge attached, so it is quite possible that there was also one at the North Gate.
"We haven't known about the possibility of a drawbridge in this location before, so it is quite an exciting discovery. By rebuilding the city for the future we are finding out more about its past."
Mr Durham said the discovery had not held up contractors.
But he added that the find was a fascinating one because it was in the vicinity of Oxford's oldest building, St Michael's Tower, thought to have been built near the time of the Norman conquest.
Darius Seroczynski, group manager for major projects at the county council, said archaeologists would keep a watching brief during the Cornmarket Street project.
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