The King is hosting a state banquet for German president Frank-Walter Steinmeier on the first day of a historic visit.
It is the first by a German leader in 27 years and the streets of Windsor have been adorned with German flags and Union flags.
Mr Steinmeier and his wife, Elke Budenbender, were met outside Windsor and Eton Riverside train station by King Charles and Queen Camilla, with the famous castle as the backdrop.
King Charles opened his arms wide, and Mr Steinmeier said: "It's so wonderful, Your Majesty, to see you again."
They took a carriage ride to the castle, where they received a ceremonial welcome.
Mr Steinmeier also joined Prime Minister Sir Keir Starmer at a roundtable of UK and German business leaders.
A glittering state banquet is taking place this evening - around 160 guests have assembled at St George's Hall, where a Christmas tree, decked with 3,000 lights, takes pride of place.
The visit comes as a £20m electric vehicle (EV) project, led by German car manufacturer Mercedes, was unveiled.
The initiative is set to create an initial 150 jobs in Northamptonshire and Oxfordshire.
In July, a partnership known as the Kensington Treaty was signed by Germany and the UK, with Sir Keir saying the countries would "work ever more closely".
'Our two countries think alike'
At his meeting with Mr Steinmeier today, the prime minister said the UK's relationship with Germany is in a "very, very strong position".
He told the president at Downing Street: "You and I sat down in Berlin last year, and I remember distinctly you saying to me that the relationship between our two countries was beginning a new chapter, a new opening. And I profoundly believe that that is right."
Read more:
NHS patients being 'set up to suffer' this winter, nurses warn
Nike, Superdry and Lacoste ads banned
Referencing the Kensington Treaty, Sir Keir added: "We've also worked very, very closely on hugely important issues like Ukraine, where our two countries think alike and act alike, on issues of migration and on economic growth and trade, where we go from strength to strength."
Mr Steinmeier will pay his respects and lay flowers at the tomb of the late Queen Elizabeth II in Windsor Castle's St George's Chapel on Thursday morning before addressing parliament later in the day.
(c) Sky News 2025: King Charles welcomes German president Frank-Walter Steinmeier to Windsor Castle for

Jeffrey Epstein's island pictured in 'never-before-seen' images released by House Oversight Committee
Doctor who supplied ketamine to late Friends star Matthew Perry jailed
'No one helped us': The Sri Lanka community left in a mass of mud and loss after cyclone
Scientist hails 'incredible find' as mysterious 'tentacles' wash up on Scottish beach