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Rochester Castle, which closed at the end of last month for safety reasons, is not likely to reopen for several weeks.
Medway Council was forced to shut the medieval fort suddenly after loose masonry fell into the historic Norman keep.
The popular tourist attraction re-opened to visitors at the beginning of July after lockdown.
The council manages the castle on behalf of English Heritage and specialists are being brought in to ensure meticulous safety repairs are carried out.
It was confirmed by the authority that a piece of mortar from around a stone fell from the 11th century structure.
Days before the closure on Thursday, July 30, a decision was made to shut the gates of the grounds after 5pm because of large gatherings congregating there ignoring social distancing guidelines.
The move came following discussions with the police and council, and security patrols were stepped up.
Officers were called on Saturday, July 18, to reports of a group of about 100 people blasting out music and partying into the night.
Meanwhile work is still under way to mend parts of a wall next to the castle at the Boley Hill car park after a lorry crashed into it.
A large section came tumbling down following the collision in May in which the vehicle mounted the kerb and hit a wall near Ye Arrow pub.
Temporary barriers have been put in place while council maintenance contractors make it safe.