Prince William and Princess Kate made the trip to Windsor on Saturday evening with Prince Harry and Duchess Meghan, Reuters news agency reported.
This is the first time the four have appeared together in public since the death of Queen Elizabeth.
It is also the first time they have been seen so close to each other since March 2020, Sky News writes.
Relations between brothers William and Harry soured, after Harry and Meghan left their royal duties and moved to the United States.
They gathered outside the castle to consider the tributes and flowers that were placed after the death of Queen Elizabeth.
According to Reuters, a spokesperson for William said he and Kate were the ones who invited Harry and Meghan.
Thousands of people had come and lined the ramparts at the entrance to Windsor.
The couple took their time to greet people. They exchanged handshakes and words with those present, similar to what King Charles did on Friday outside Buckingham Palace.
There were many who were prepared with bouquets of flowers of various sizes. Many of those present also received good hugs.
For more than half an hour, they went from person to person along the fences and greeted each other, before all four boarded a black car that set off with Prince William behind the wheel.
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