THE owners of Richmond Castle are appealing to the public to help them repel teenage "invaders" who are breaching the stronghold's defences and vandalising the ruin.

It is not a new problem and English Heritage staff admit trespass in the grounds has always been common.

However, recently, unofficial visitors have been showing less respect for the market town's heritage with the amount of damage on the increase.

A bench was hurled over a 20ft drop from the keep and left smashed to pieces on the gravel path below.

Smashed bottles have been left strewn around the grounds prompting concern that a shard missed during the morning clean-up could injure a visitor.

However, staff say they are equally concerned that, if the night-time visits continue, they could find themselves dealing with something even more serious.

"There are some very steep drops from the castle walls and I dread to think what could happen if someone who has had a little too much to drink was to fall," said head custodian, Nick Woodward.

"Someone's family could find themselves dealing with a tragedy and we just don't want that to happen."

English Heritage has considered improving security around the castle perimeter, although the measures that can be taken are limited because of the castle's importance as an historic monument.

"It is ironic in a way, as the castle was built to keep invaders out, but we don't want to add anything that would spoil the setting both for Richmond residents and for visitors," said Mr Woodward.

Police have been alerted to the problem, although, so far, the culprits have managed to get away before patrols responding to complaints have had time to get into the grounds.

"They have keys and we know they are keeping an eye on the castle, but there is only so much they can do," said Mr Woodward.

"What we really need is for anyone who sees or hears anything in the grounds after the gates have been locked to call the police immediately.