Manx National Heritage says it's shocked and disappointed after extensive graffiti was discovered at a historic site near Castletown.
The walls of a post-medieval summerhouse at Hango Hill have been spray-painted, with the incident now reported to police.
The protected ancient monument dates back to the Bronze Age and was a place of execution during the Viking Age.
MNH says it's assessing the damage and anyone who saw anything suspicious or has information is being urged to contact police.
New Housing Minister focuses on Island's housing needs
Douglas South ward vacancy filled without a vote
Douglas to hold Remembrance Sunday parade and service
MHK to question Treasury Minister over extra fuel tax revenue
Police investigate sex assault claim against officer
FOI request queries Douglas Head road repairs
Company launches Xmas gift donation scheme for ill children
Living Hope applies to return Castletown building to church use