Plans by Queen Elizabeth's estate to convert disused stables into a house within the boundary of a Doncaster castle have angered residents.
The Duchy of Lancaster also wants to renovate and extend a cottage within the grounds of Tickhill Castle.
But the proposals, which are recommended for approval by Doncaster Council's planning team, have faced dozens of objections.
Residents said the site is of "national interest" and should be preserved.
In a letter to planners, Graham Smith said: "If the castle was in any city in the south, this would never happen.
"The castle and its environment must be preserved for future generations."
Patrick Doran, who lives close to the castle, said he protested strongly against the planned development.
'Back to life'
"The proposed building is on land which houses an ancient monument of significant local and national interest. We feel it is highly inappropriate to even consider developing new build properties in and around the area that houses the castle monument," he said.
"The proposed development is in the heart of an existing green belt area and therefore wholly inappropriate for the development of new buildings."
But Historic England has supported it on "heritage grounds" and "remains supportive" of the plans to bring the farmhouse back into use, the Local Democracy Reporting Service has reported.
Castle Cottage is thought to be an unlisted two storey farmhouse with attached single storey stables and outbuildings at the foot of Tickhill Castle.
It is located along the line of the historic ditch surrounding the motte.
Gareth Stent, the authority's principal planning officer, added the proposals would not be harmful to neighbours and would bring "no significant harm to the character and openness of the Green Belt and no significant harm to the heritage asset".
The Duchy of Lancaster provides the monarch with an independent source of income and administers almost 19,000 hectares of land across England and Wales.
It has been approached by the BBC for comment.