Two scheduled monuments have been damaged by vandals, prompting police to carry out patrols of the areas.
Dyfed-Powys Police said former bomb stores in Pembroke Dock had been defaced with graffiti and and Iron Age site Bowett Wood Camp near Hundleton had been dug up to create ramps for off-road bikers.
It said the patrols in in Pembrokeshire were intended to deter further damage.
Anyone with information has been asked to contact the force.
The force said it was not clear when the damage to the former bomb store was caused, and appealed for anyone who might recognise the design or have information about who painted it to come forward.
Insp Reuben Palin, force lead for heritage crime, said: "Unwanted visitors and antisocial behaviour are a problem at the bomb stores, and someone has taken it on themselves to cause a significant amount of damage by painting a design on one of the walls.
"This site might look like nothing more than a derelict building, but is in fact a well-preserved relic with significant historic value."
He added: "I'd like to remind people that Bowett Wood Camp is an ancient monument and even with the landowner's permission to enter, any disruption and damage to the area is a criminal offence."