Kelburn Castle is covered with a psychedelic series of interwoven cartoons painted by a group of Brazilian graffiti artists in 2007.
Despite protests, it was announced in 2012 that the mural is to be removed after engineers discovered the cement coating on which it is painted is causing damage to the original castle walls, but work has yet to start.
Owned by the Earl of Glasgow, the castle near Largs, North Ayrshire, was closed in 2009 after a fire in the roof. Extensive renovations have now been finished and it is to reopen at the start of April as a venue for weddings, parties and corporate events.
The work is part of a £2.4 million investment in the Kelburn Estate to increase visitor numbers to the country park.
It was funded by the sale of 18 acres of land at the rear of the estate to a housing developer. Log cabins and teepees are also planned on the estate to add to a caravan park already on the grounds.
The current Earl of Glasgow, Patrick Boyle, said: "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.
"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."
The 13th century castle contains a historic collection including a gown worn by the Lord Chief Justice of Scotland at the trial of Burke and Hare and a chair used at the coronation of the Queen.
The mural is a popular feature and was previously named as one of the world's top 10 examples of street art, on a par with Banksy's work in Los Angeles and the Favela Morro Da Providjncia in Rio de Janeiro.
Work to remove it from the castle is expected to start within the next year.