Archaeologists who set out to put up a safety fence at Rochester's medieval castle have unexpectedly uncovered a Roman city wall.
Archaeologists planned to build a fence but found a Roman wall
The team had "barely taken the turf off when they unearthed a solid mass of stone masonry", Medway Council said.
Castle archaeologist Graham Keevill called it "a very important discovery".
He said: "We don't have many Roman city walls surviving in England. To get an unexpected one like this is fantastic. It is also a perfect example."
He said the wall had "high-quality" facing stones on each side, and its rubble core, made up of stone, flint, sand, and gravel, would have been poured in "to set hard almost like concrete, to bind the whole wall together".
Builders who came later in the 12th Century "knew good masonry when they saw it" and used the 6ft-wide (1.8m) Roman wall for the foundations of their medieval castle keep, Mr Keevill said.
The pits will be re-covered to preserve the find, and the safety fence will be realigned.
The rubble-filled wall is 6ft (1.8m) wide with high-quality stonework
It is the second time Mr Keevill has unexpectedly discovered Roman remains.
At the Tower of London, he was part of the team that found a city wall of Roman Londinium, that had been re-used in the foundations of a medieval tower.
"It's an amazing coincidence," he said.
The work under way at Rochester Castle is part of a conservation project by Medway Council and English Heritage to repair the ramparts and some stonework, fit new balustrades, and put up a new safety fence.
According to the council, the Romans built their fort next to the River Medway to guard the bridge carrying their legions from Dover to London.