About £175,000 is to be spent on urgent repairs to Newark Castle.
A report by an expert building conservation company has identified high priority repairs that are needed on the walls of the 12th Century monument.
Two areas of the castle gardens have been fenced off to protect the public from risk of falling stones.
Newark and Sherwood District Council, which is responsible for the castle, has decided to allocate £130,000 towards the repairs. The rest of the money will come from a heritage grant via the Landfill Communities Fund.
Further repairs have been identified that are less urgent. Some will be carried out within two years and others within five years, depending on their priority.
The total cost of all the repairs required is about £1/2m.
The district council’s cabinet member with responsibility for the castle, Mrs Nora Armstrong, said: “We can’t do anything other than look after this wonderful asset.
“It just can’t be allowed to deteriorate.”
The council leader, Mr Tony Roberts, said it was an important part of the history of the district.
He said it was a concern that over the years it was evident that plants had started growing in the mortar at the top of the walls. He said regular yearly inspections were needed.
The castle was partially destroyed at the end of the Civil War siege of Newark in 1646, exposing soft blue lias stone, which has been refaced in some areas but not others.
Areas of the riverside curtain wall and gatehouse tower have been particularly affected by weathering, resulting in small pieces of stone frequently falling to the ground.
The district council carried out extensive restoration work costing nearly £1m between 1979 and 1994.
The majority of the money for that work came from English Heritage, which is now considering whether to increase the risk status of the castle from low to medium.
This would open up the possibility of the castle receiving a grant, although restrictions in funding due to the Government’s spending review mean it would not be guaranteed.