The Sussex Archaeological Society said it is ‘saddened’ by today’s incident at Lewes Castle.
Debbie Matthews, marketing officer for the society, which protects the heritage of Sussex, confirmed a privately-owned piece of the castle wall had collapsed.
She said the castle was not affected but had been closed to the public for safety reasons.
Emergency services remain at the scene, which has been cordoned off by police.
Ms Matthews said: “It is a private piece owned by one of the local residents – it is part of their garden wall. Sadly, part of it has collapsed today causing significant damage.
“It has partially fallen into someone’s garden and against the Old Coach House.
“The society is obviously saddened by the collapse of this 1000-year-old piece of history.
“The sad incident on 11th November of the collapse of a privately owned section of Lewes Castle’s curtain wall has led to the decision to close the castle on 11th November whilst an inspection of all the walls is carried out.
“This part of the wall is a separate piece away from main castle structure and is a precautionary measure due to the recent incident and adverse weather conditions.
“The museum remains open and the castle is expected to reopen on 12th November.
“The parts of the castle owned and cared for by The Sussex Archaeological Society are annually inspected by a third party expert and regularly by staff to ensure the safety of our visitors, staff and members of the public.”
Ms Matthews confirmed no injuries had been reported. She said the area had been sectioned off by the emergency services.
She added: “We are very sad. The Sussex Archaeological Society’s aim is to protect the heritage of Sussex. It is sad, a piece of history has gone.”
The Sussex Archaeological Society began in 1846 and is the oldest archaeological society in England.
It is a registered charity and its charitable aims are to enable people to enjoy, learn about and have access to the heritage of Sussex.
It does this by opening six historic sites in Sussex to visitors old and young, providing research facilities in its library, running excavations, providing a finds identification service and offering a variety of walks, talks and conferences on the archaeology and history of Sussex.
The charity relies on income from admissions, donations, grants and membership to enable it to look after the buildings and sites in its care. It does not receive any any government funding.