English Heritage says it is a 'terrible blow' to Hurst Castle - after its seawall collapsed.
There were no injuries during the incident yesterday (Friday).
The damage is being assessed by specialist teams.
English Heritage, which owns the historic building, said:
"Everyone’s safety is our first priority and there are no reports of injuries. The castle was closed.
"It’s a terrible blow for this great site and all who care for it.
"Our engineering team is there now, assessing the damage and next steps."
The castle - built by Henry VIII -formed part of the king's Device Forts coastal protection programme against invasion from France and the Roman Empire.
The castle was used as a prison for eminent 17th-century captives, including Charles I.
It was later strengthened during the 19th and 20th centuries and played a role in defending the western Solent from invasion threats from the Napoleonic Wars to the Second World War.
Sad to see that the seawall of @EnglishHeritage Hurst Castle has collapsed. Fortunately nobody got injured. pic.twitter.com/N4BB5DK0mc
— Quiz Britain 🇬🇧 (@Quizbritain) February 26, 2021
Thank you, everyone’s safety is our first priority and there are no reports of injuries – the castle was closed. It’s a terrible blow for this great site and all who care for it. Our engineering team is there now, assessing the damage and next steps.
— English Heritage (@EnglishHeritage) February 26, 2021
Read more about the site here.