People have damaged the structure of Bronllys Castle at Talgarth by throwing historic stones off the top, says Welsh monument agency Cadw.
The castle only recently re-opened following a major conservation programme.
Cadw has informed police but say there may be no choice but to close the castle.
The late 11th or early 12th Century motte with an 80ft tall round masonry keep has had a number of ancient stones prised from the top and thrown down both inside and outside the structure.
As a short-term measure, the access door to the top of the motte is to be closed for a time to enable repairs to be undertaken and the stones replaced.
A recent works programme included improvements to the castle’s main entrance and “robust” new metal access stairs to the tower.
The castle was built by anglo-Norman lord Richard Fitz Pons and in the 13th Century, a garrison defended the fortress against Llywelyn the Great.
The public were also given access to the wall-walk level and a secure platform installed to offer visitor views in all directions.
Wayne Evans, Cadw’s Visitor Services Manager, said, “Closing a monument is always the last resort, but if vandals are determined to continue to damage the castle we have no other choice.
“The health and safety of our visitors is paramount and while we cannot allow the actions of a very few to spoil the enjoyment of others we cannot risk visitor safety.
“We have informed the local police and hope to track down those responsible as a decision to close the monument is in nobody’s interest.”
Source: BBC News