First of all, let's clarify what we mean by "Narnia." Technically, Narnia is a country – the nation ruled by King Caspian. In the world where Narnia exists there are also other countries, such as Calormen and Telmar, as well as Narnian territories like the Lone Islands. However, sometimes people use the word "Narnia" to refer to the whole magical world which the country of Narnia is a part of. It's not just critics and fans who do this – it's also characters in the books. This becomes more complicated when we're thinking about The Voyage of the Dawn Treader, which takes place entirely in the seas east of the country of Narnia and on islands in those seas. Nobody in this book actually sets foot in Narnia the country, but they are in the "world of Narnia" – the world that has Narnia in it, as opposed to what Lucy, Edmund, and Eustace refer to as "our world," the world of mid-20th-century England.
In this book, the characters visit a variety of islands, including the Lone Islands (Felimath, Doorn, and Avra), Dragon Island, Burnt Island, Goldwater Island, the Island of the Voices, the Dark Island, and Ramandu's Island. Except for the Lone Islands, which are Narnian territories with villagers, markets, and governmental structures, all of the islands are fantastic, unknown places that the adventurers themselves name and put on the map for the first time. Each island is like a small, self-contained world with its own mysteries and dangers. Although the crew of the Dawn Treader encounters magicians, fantastic creatures, and supernatural occurrences on the islands, the islands themselves are relatively tame and uninteresting. The vegetation on the islands looks vaguely like the plants and trees that Lucy, Edmund, and Eustace know from their own world – for example, on Ramandu's island:
Underfoot was fine springy turf dotted here and there with a low bushy growth which Edmund and Lucy took for heather. Eustace, who was really rather good at botany, said it wasn't, and he was probably right; but it was something of very much the same kind. (13.4)
Similarly, when Eustace is a dragon, he uproots "a great tall pine tree" (7.14) for the Dawn Treader's new mast. So what happens on the islands may be fantastic, but the plants and landscapes are close to that of our own world. It's interesting to note that C.S. Lewis is more interested in the development of the characters and in the fantasy elements of the story than in creating an truly unique setting. In this sense, the islands and countries that the Dawn Treader encounters are reminiscent of the strange lands in Gulliver's Travels – the countryside itself is not unusual, but the people are.