Video report by ITV News Royal Editor Chris Ship
The monarch’s appearance was the first time she has been seen in public with the Prince of Wales and senior members of the monarchy en masse since before the coronavirus pandemic.
William and Kate’s trip has provoked veiled criticisms from Welsh and Scottish ministers, who raised suggestions about the timing of the visit while Covid cases were still prevalent and many parts of the UK were subject to strict coronavirus rules.
But Prime Minister Boris Johnson said he believes William and Kate’s tour, which has covered 1,250 miles over three days, was a "welcome morale boost", after Downing Street officials initially refused to say it complied with coronavirus restrictions.
In wintry conditions, the Queen stood on the steps of the Equerry’s entrance in the castle’s quadrangle and listened as the Regent Hall Band of the Salvation Army, who are based in London’s Oxford Street, played Christmas Carols.
Socially distanced around her were Charles and the Duchess of Cornwall, William and Kate, the Earl and Countess of Wessex and the Princess Royal – who had all arrived separately to the monarch.
Earlier, Welsh Health Minister Vaughan Gething had said he would prefer it if “no-one was having unnecessary visits” before William and Kate travelled to Cardiff Castle and met students.
As they chatted, William and Kate admitted they were struggling with Christmas plans, suggesting they had yet to decide who to spend the festive period with.
“It is so difficult, we are still trying to make plans. It’s difficult to know what to do for the best,” the Duke said.
William and Kate have been touring the country thanking key and frontline workers and communities for their efforts during the pandemic.
On Tuesday, they began the day in Cardiff, before travelling to chat to staff and briefly meet residents at Cleeve Court Care Home in Twerton in Bath.
Their penultimate stop before Windsor was the Royal Berkshire Hospital in Reading, to pay tribute to the work of the nurses.
On the day NHS began its mass vaccination programme across the UK, William told the medical staff: “Keep going. There is light at the end of the tunnel.”
Mr Gething said he was not “particularly bothered or interested” when asked during BBC Radio 4’s Today programme if he thought the couple should travel to Wales.
But he said William and Kate’s visit should not be used by people as an excuse to say they are “confused” about coronavirus regulations.
Mr Gething echoed the sentiment of Scotland’s First Nicola Sturgeon, who suggested the Duke and Duchess travelled to Edinburgh on Monday despite their office being made aware of restrictions for those wanting to cross the border.
In response to a suggestion that No 10 was refusing to give its backing to the couple’s trip, the prime minister’s official spokesperson said: “I would point you towards the palace.”
Later a statement was issued confirming the Mr Johnson’s support, a No 10 spokesperson said: "The PM is delighted to see the warm reception the Duke and Duchess of Cambridge have received on their hugely valuable train tour of England, Scotland and Wales.
"The tour will be a welcome morale boost to frontline workers who have done so much during the pandemic."