Work has begun to clear rubble from Guernsey's Fort Hommet so a survey can be carried out on the site.
The Culture and Leisure Department has undertaken the work on the fort, which dates back to the 1600s and was also fortified by the German forces in WWII.
Clive Martin, museums and monument manager, said the site was in urgent need of conservation work.
He said the work was likely to take at least three years and would not discuss the potential cost.
Mr Martin said: "We're not looking to restore the site by putting guns back or any other historical detail, we're only concerned with preserving the actual structure."
The museums service said it had just about enough funding money for this project, but it would like an extra £150,000 a year for the next five to ensure all the island's monuments get a 21st century make over.