The realms of the Elves in Middle-earth are their last bastions in this kingdom. The Elves are in fact about to leave the world and retire to the countries beyond the sea. Originally, humans could also travel to these countries, but they have slowly pulled further and further back, and it is no longer possible for humans to get there under their own power.
In medieval literature, the realm of the elves is described as a parallel world alongside the human world. It is a wonderful but dangerous place. Tolkien himself writes:
“The Elves’ world is wide and deep and high and filled with many things: all manner of beasts and birds are found there; shoreless seas and stars uncounted; beauty that is an enchantment, and an ever-present peril; both joy and sorrow as sharp as swords. In that realm a man may, perhaps, count himself fortunate to have wandered, but its very richness and strangeness tie the tongue of a traveller who would report them. And while he is there, it is dangerous for him to ask too many questions, lest the gates should be shut and the keys be lost.”
For Tolkien, this was a realm that actually existed. A realm that exists in the common intellectual thinking and which is constantly expanded or developed when the author further brews existing stories and their elements. Tolkien’s universe is on the one hand created, but also a further development of the mythological heritage which helps to keep it alive. It is part of human nature and the “more precise and clear the reason is, the better the fantasy world it creates”.