Emergency teams with sniffer dogs are searching for casualties after a wall at a 1,000-year-old castle collapsed.
About 600 tonnes of stonework and other materials at Lewes Castle in East Sussex fell on to a house and garden.
Emergency services, including a hazardous area response team, are at the scene of the collapse of the 10m (32ft) by 10m structure.
Sussex Police said it was not known if anyone was trapped, adding: “There are no current reports of any injuries.”
Sniffer dogs trained for use in earthquake zones have been deployed for the search.
East Sussex Fire and Rescue Service was called to the scene at 12.22pm, along with search and rescue staff.
Incident commander Matt Lloyd said the wall had collapsed into an adjacent garden and house.
The wall was reportedly undergoing work to remove ivy in recent weeks.
A fire service spokesman said: “Firefighters are working alongside other emergency services to check the area to ensure that no one needs rescuing.”
A spokeswoman for castle owners Sussex Archeological Society said the wall that collapsed was privately-owned and the section that had fallen was part of the curtain wall that ran round the entire outside of the medieval landmark.
It was one of the last remaining sections of the historic defence wall, she said, adding: “There’s not much of it left standing. It’s one of the last sections that was still at full height.”
Staff at the castle closed the attraction on Monday as a precautionary measure, the spokeswoman said.
She said the castle walls that are owned by the archaeological society are checked independently on an annual basis.
Several parts of the castle structure are privately owned by different property owners.
Lewes District Council said officers from its building control team were at the scene and liaising with emergency services.