Lindisfarne Castle has shut to the public for £3 million worth of repairs that will last for 18 months.
The 16th Century castle, built in 1550 and now owned by the National Trust, is located on a headland on Holy Island, off the Northumberland coast.
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As well as issues of damp, the stonework, painting and windows will also be treated by experts in works that will keep it closed to tourists until April 2018.
Addressing the issue on its website, the National Trust explained: "The issue at Lindisfarne has always been (and will always be) the environment. High on Beblowe Crag with commanding views of the coastline and sea, it is easy to see why a military building was constructed here. But these enviable features also carry a flipside, with the exposed situation meaning the weather is a constant problem for the building.
"Where there have been problems here in the past, the owners (ourselves included) have tended towards 'quick-fix' solutions. What sets this current work apart is that we plan to run a series of trials around the building; different methods, different materials, different places. After a period of exposure to the weather, we should know what works best and where.
"The idea behind the work is to try out a number of methods designed to combat the problem we have at the Castle with water ingress. We have leaking windows, issues with guttering and downpipes, and moisture getting in through the walls (and not going out again).
"Certain materials used in the past during maintenance work are hindering the movement of moisture through the structure, and so causing problems in certain areas.
We have identified a number of key areas where particular methods can be trialled, monitored and evaluated. This will then feed into a major project in 2017 when these methods can be rolled out throughout the building."
Speaking to the BBC, a National Trust spokesman said: "We had hoped to have the castle open to visitors during the project works, but as we started undertaking the trials it became clear that the amount of interruption and intervention by contractors would not be compatible with visitors in such a small confined space.
"We considered a number of partial opening models, but each we felt would only prolong the work beyond what is reasonable. "