Damage has been caused to the ruins of Narberth Castle.
During half-term week and over the weekend, children and youngsters entered the grounds and caused damage to the door of a portable building used by the workmen.
In addition, all the yellow plastic warning signs surrounding the area, warning members of the public about the dangerous structures and advising them to keep out were either damaged or removed.
Privately-owned, Narberth Castle has, until recently, never received any public funding, but over the last few years the remains have become extremely unsafe.
Pembrokeshire County Council has taken a 99-year lease on the castle and have successfully applied for funding from various organisations for access improvements, repairs etc.
Picton and Tree Ltd., of Milford Haven, are the contractors at the site and scaffolding has been erected around the dangerous structures.
The workmen have erected signs around the site, together with steel and wire security fencing, but due to the nature of the site, it is impossible to close off the site 100 per cent.
"Young children have been seen climbing over the fencing and up the scaffolding, which at its highest point is approximately 50-foot in height," said PC Dorian Evans, of the town's police station.
"Wooden planks have been thrown from the scaffolding and youngsters have been seen, by local residents, swinging on the security ropes," he explained.
"Although there is concern for the vandalism, the main worry is the danger that these children and youths may cause to themselves.
"We wish to make parents aware of what is going on and ask that they warn their children of the dangers and possible outcome if one were to fall from this height," he added.
Local residents have been asked to contact the police if and when they see any trespassers.
Anyone with any information about these incidents is asked to contact PCEvans at Narberth police station on 860389.