Vandals are believed to have damaged stonework at Vale Castle over last weekend.
Guernsey Police said the incident happened on 11 or 12 April.
It is the second reported criminal damage at the site in recent weeks.
Both times “sections of wall capping, the protective stonework placed along the top of the walls, were removed, causing further damage to the structure,” officers said.
Anyone with information should contact PC 146 MOORE on 222 222 or Crimestoppers anonymously on 0800 555 111.