Duchess Camilla called herself an "ancient Fairy Godmother" as she spoke at the BBC's 500 Words competition.
The 69-year-old royal attended the event to celebrate the successes of the young writers up and down the country.
She said: "Now I suspect, Ladies and Gentlemen, that you never thought you'd be sent to The Tower of London ... but here we are today! And if you've read the 'Horrible Histories' you'll know that, in the old days, it was usually a one-way ticket!
"As I look round at the happy faces today, I think I can safely say that we are all very excited to be within these historic walls. Chris Evans and the team at BBC Radio 2 have planned yet another amazing extravaganza to celebrate the final of this magical 500 Words competition, and as I was cast as its ancient Fairy Godmother I'm hoping that there's still a little bit of magic floating around ...
"But we don't really need extra magic... look where we are! The Tower of London is such a famous landmark and such a powerful symbol of London."
And Duchess Camilla - who is married to Prince Charles - wants to encourage more young people to pick up a pen and write.
She added: "But today is all about your stories. I've read some of the brilliant ones that you have written this year, so I know that a rainbow of different ideas has stimulated your imaginations. I'm sure you all know, better than me, that inspiration can play tricks on you. One day you can sit for hours and wait in vain ... and the next it can come straight out of the blue. It might begin with your own memories ... happy or sad, or snatches of overheard conversations, or even glimpses of people brought to life by visiting places, drenched in history, like this.
"Wherever you find your inspiration, in the past, present or future, I'm thrilled you love writing. Your stories are brimming with ideas, and fizzing with energy, and hopefully encouraging others, who are listening today, to follow your example."