STONEMASONS with a head for heights are preparing to scale Edinburgh Castle to repair towers for the first time in half a century.

Historic Scotland is carrying out the work after discovering that years of wet and stormy weather have taken their toll on the stonework of two sentry boxes.

Scaffolding has been installed around the landmark for the maintenance work, which is due to take up to three weeks.

Experts will brave sheer drops from the steep south side of the castle to complete the repairs. The workers will also abseil down Castle Rock to remove plants and trees, which can threaten the stability of the rock.

David Storrar, regional architect for Edinburgh Castle, said that eroded lime mortar had been spotted during a routine inspection of the castle last year.

Two sentry boxes on the western defence walls, overlooking the west end of Princes Street Gardens, have become damaged. One of them is known as the Queen's Post, while there the other sentry box needing patched up is located by the One o'Clock Gun.

The boxes, which date from around 1730, were positioned at intersections to allow surveillance of any enemy approaches to the castle's outer walls.

Mr Storrar said: "This work will last 50 or 100 years, depending on the exposure. Hopefully people will not be able to notice the difference, as the gaps in the stonework are very narrow.

"This is essential work, especially when it is overlooking the railway. The castle is checked two or three times a year, and they are especially careful in case there is anything that could fall off."

It will take around a week to repair each sentry box. The work is being carried out by stonemasons from Historic Scotland's special monument conservation unit.

Mr Storrar said: "The scaffolding has had to be self-supporting so we don't have to take it down the rocks.

"They're fully trained, but it's not risk free. The precipitous nature of the castle rock and sheer drops make the work more interesting, and the views across Edinburgh are wonderful - but only if you have a head for heights."

Mr Storrar said they were also carrying out work on the castle rock. Workers will scale the sides wearing special harnesses.

He added: "They need to take up trees and plants which can destabilise the basalt rock. It's already got quite a fractured appearance.

"Most of the work tends to be done just after the summer season. We try to maintain its appearance during the summer as much as possible."