Kelburn Castle, which has been in the one family since 1350, has undergone a £500,000 investment which the owner, the Earl of Glasgow, said would "consolidate its status as a significant tourism and hospitality facility".
The castle in Largs, Ayrshire, which has eight bedrooms available for public use, was closed for residential use in 2008 but major works have now been carried out on the building, including money spent on installing external fire escapes and fire doors. But the building's famous mural, which was completed by a group of Brazilian artists in 2007, is to be removed after engineers discovered that the cement coating on which it is painted is causing damage to the original walls.
The Earl of Glasgow, Patrick Boyle, said: "We have some very exciting plans for the castle, which now must be the safest in Scotland, as well as for the estate which we are confident will help to make it one of the best tourist attractions, not only in Ayrshire, but in the whole country.
"This is the biggest investment in Kelburn Castle and Estate for a century and will consolidate its status as a significant tourism and hospitality facility."
Experts have said the stonework beneath the graffiti is beginning to deteriorate because of the cement render which was applied in the early 1950s.
The mural is expected to be removed within the next year.
The residential project is part of a £2.4 million investment in Kelburn Estate, which was funded by the sale of land for housing.
It includes a caravan plan, with further plans to build log cabins and yurts on site for holiday-makers.