Underground passages were built after the castle proved vulnerable in 1216
A £250,000 project to restore brickwork at Dover Castle has begun.
English Heritage said experts were working to stabilise a maze of brick-built passages, and repairing the roofs and details on the eaves.
Scaffolding has been put up in ditches at the medieval structure, but tunnels dating back to the Middle Ages remain visible, the charity said.
This summer, English Heritage unveiled a £2.45m recreation of Henry II's 12th Century opulent interiors at the site.
Work is taking place on the brickwork of the defences at the northern tip of the castle, known as the Spur.
Steve Lang, head of visitor operations, said: "This conservation work is necessary to halt collapse of some of the unstable brickwork."
He said it was also to ensure this unique part of Dover Castle remained in good order, so it could be enjoyed and studied by generations of visitors to come.
According to English Heritage, the castle has been heavily fortified since the early 13th Century, when Henry III created earthen defences with masonry towers and underground passages.
The castle had proved vulnerable in 1216, and the tunnels allowed soldiers to cover the structure's perimeter defences.
Further construction of underground passages known as caponiers took place during the Napoleonic War, English Heritage said.
Repairs are being carried out by a London firm which specialises in the repair of historic brickwork.