On other holidays, roaming unaccompanied on my bicycle near my grandmother’s house in the English countryside, I used to play on the top of an Iron Age hill fort called the “Trundle.” The view was extraordinary — I could see for miles and miles — and I had a strong feeling of what J.R.R. Tolkien called “the heart-racking sense of the vanished past.” It made me wonder: Who were the people who lived here, long ago? What were their lost histories?

From the hill fort, I had a good view of Levin Down, an extraordinarily eerie, wondrous place that we called the “Fairy Hill.” It stood out from the quiet country fields around it in an ominous and striking fashion, like the back of a whale surfacing in the ocean. The trees had blown there by chance, rather than being planted by human hands, and some were yew trees: I was warned not to touch them. There were places I could not go and “Beware” signs (which I always found exciting) because the area had been used as a training ground during World War II. To a sensitive child like myself, it was easy to believe that this hill was enchanted. It was intriguing, eerie, exciting and beautiful all at the same time.

The hill was covered with strange grassy mounds about the size of molehills. The adults had no idea what they were — which was very exciting to me, realizing that there were things in the world that not even the adults understood. So I filled in the blanks for myself and decided they must be burial mounds for fairies. This was the magical landscape that inspired my book “The Wizards of Once.”

For the wildwood in that book, I took particular inspiration from the ancient wood of Kingley Vale in Sussex. Its trees have gnarled, expressive faces, and roots that embed into the earth with an almost visceral power. The more you learn about trees, the more magical you realize they are. Did you know, for example, that trees can communicate with each other through their roots, even when they are many miles apart?

Trees grow throughout children’s books. From “Peter Pan” to “A Monster Calls,” “The Lord of the Rings” to “Harry Potter,” trees are refuges, prisons and symbols of nature’s potency. They can be a friendly home, like the Hundred Acre Wood in “Winnie-the-Pooh,” or give a sense of menace, like the snowy forest in “The Lion, the Witch and the Wardrobe.” They can also be symbolic, like the cement-filled dying tree in “To Kill a Mockingbird.” The writers I loved when I was a child were similarly inspired by magical landscapes and nature: Ursula K. Le Guin, J.R.R. Tolkien, L. Frank Baum, Diana Wynne Jones, Lloyd Alexander, Robert Louis Stevenson, T.H. White — and so many others.

Today, children have much less unsupervised access to the countryside. I worry that they may never know the magic of the wilderness, the power of trees and the thrilling excitement of exploring nature without an adult hovering behind them. And so I write books for children who will never know what the freedom of my childhood was like.

My father never found out whether the nest in the cliff belonged to a white-tailed sea eagle or a buzzard. The gust of wind that scared me brought him to his senses, and he scrambled back up the side of the cliff before I could lose my grip. We staggered back through the gale to the little stone house on the island where, in the candlelight, we dried out in front of the fire.

Cressida Cowell (@CressidaCowell) is the author and illustrator of the “How to Train Your Dragon” and “The Wizards of Once” series of children’s books.

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On other holidays, roaming unaccompanied on my bicycle near my grandmother’s house in the English countryside, I used to play on the top of an Iron Age hill fort called the “Trundle.” The view was extraordinary — I could see for miles and miles — and I had a...