The activation was designed to promote Steven Spielberg's forthcoming film, The Big Friendly Giant, an adaptation of the children's book by Roald Dahl.
Measuring 10-feet tall, the cake was layered with the character's favourite 'scrumdiddlyumptious snozzcumbers' over four rather wonky tiers.
Premier designed it to mimic the brithday cake given to the Queen by Nadiya Hussain, who won The Great British Bake Off last year.
After passing Buckingham Palace en route to the castle, the cake was delivered to Windsor Castle by two people dressed as foot soldiers, on a table covered with a Union Jack flag.
It was accompanied by a card that read 'Happy birthday your Majester, Yours The BFG.'
In the books, the BFG visits Buckingham Palace with main character Sophie for a breakfast with the Queen featuring giant cutlery and furniture.
The film is due to be released on 22 July, with 2016 also marking the 100th birthday of Roald Dahl.
For more imaginative brand activity around the Queen's 90th birthday, check out our roundup here.