Blyth firm Team Force Restoration is carrying out conservation work on the 300-year-old walled garden at Auckland Castle in County Durham.
The job, due to be completed this month, follows the £17m restoration of the castle, home to the prince bishops of Durham for 900 years.
According to an archaeological survey last year, the two-acre walled garden site dates from the 1670s, but its present walls were only built in the 18th century.
Over recent years, the garden had become derelict.
Scaffolding has been put up, and some stretches of wall affected by ground subsidence have had to be propped up.
A 10-strong team of craftsmen from the Quay Road firm have also had to cope with the added complication of working alongside a colony of bats.
Team Force Restoration managing director Brendan Teasdale said: “We are privileged to be part of this significant project.
“The consolidation work we are carrying out to these historic walls will safeguard them for future generations.
“Careful consideration is taken when repairs to this type of decayed masonry are carried out because under no circumstances do we want to change the overall appearance.
“As always with heritage restoration, the work is very detailed, involving meticulous research, as well as brick-by-brick recording and then careful dismantling where the walls have subsided.”
“We have also replaced stone copings, rebuilt foundations, replaced bricks and stone, installed reinforcement, inserted ties and carried out repointing and consolidation work where cement mortars had been removed.
“Most historic properties we work on need intervention. However, we don’t want to change the aesthetics of a building unless it is absolutely necessary for public safety, so when bricks are replaced, it is only when necessary, ensuring that any repairs are kept to the minimum.
“It is also important that the original bricks still show signs of erosion, demonstrating the history of the wall.”
Phil Bolam, head of estates at the castle, added: “We are very pleased to be working with Team Force Restoration on this the first stage of the walled garden restoration.
“After many years of neglect, the walled garden was in need of a great deal of care and attention.
“Team Force Restoration’s skills in accurate restoration were ideally suited to bring the garden wall back to its original 17th century characteristics.
“The position and layout of the garden added to the challenge, with steep terracing and almost unpassable terrain.
“The team are coping with professionalism and skill to ensure the work is completed on time.
“Their understanding of historic accuracy in materials and craftsmanship ensure that this feature will be sympathetically restored.
“This is the first stage of a very exciting restoration and build project. Without this initial restoration work, the project could not progress, and we look forward to beginning the second phase of work in May 2016.”
Planning permission was granted in April 2015 for the £17m restoration of the castle, a key step in the wider £60m renovation of one of Britain’s most important historical sites, being redeveloped by the Auckland Castle Trust as a new heritage destination.
The castle, once the work is complete, is expected to welcome 120,000 visitors a year, creating more than 100 full-time jobs and generating £3m in annual revenue.
Team Force Restoration, founded in 2002, has worked on more than 150 historical and ecclesiastical buildings for clients including the National Trust and Historic England.
Sites it has previously worked on include Seaton Delaval Hall, Lindisfarne Priory, the Coquet Island bird sanctuary, Beadnell’s lime kilns, St Matthew’s Church in Newcastle and the Black Barn, near Bardon Mill.