Fort Doyle in the Vale has been vandalised, with various stones ripped up and thrown around the area.
It happened sometime between 16 July and 29 July - when the damage was noticed.
Two of the stone steps were thrown onto the rocks at the side of the Fort, while bits of stone from the roof were torn off and disregarded along with stones taken out of the walls.
"It's always very disappointing to occurring on protected monuments," said Historic Site Curator for Guernsey Museums, Helen Glencross. "The vandalism was reported to me by one of our wardens on Monday. It's so disappointing because there's obviously the staff times and the staff costs."
Guernsey Police is appealing for information, and anyone who may be able to assist with the investigation is asked to contact PC 129 on 725111 or Crimestoppers anonymously on 0800 555 111.
A stonemason is being hired to repair the damage.
Pictured top: Fort Doyle.
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