The Description of a New World, Called The Blazing-World by Margaret Cavendish, printed first in 1666 and reprinted in 1668.
The protagonist, a woman, travels to another world at the North Pole. You have to go to the actual pole, and try to travel trough it, in order to reach the other world. There's a comment by the author that suggests there are many worlds ("skilful Astronomers have often observed two or three Suns at once.")
There's no small doorway, but this link between worlds does allow for seeing from one into the next.
This other world is clearly defined as being a separate world (even with its own sun, which moves differently in the sky than ours). It also has strange creatures (such as Bear-Creatures and Fox men) that aren't the typical Fey creatures from folklore. This other creatures even have their own language, which the Lady learns.
It's not as concrete of an example as in Phantastes, where different doorways lead to different worlds, but it does show us that writers were thinking of ideas such as travelling to unique new worlds at least as early as the mid-1600s.
Travel to the other world:
Neither was it a wonder that the men did freeze to death; for they were not onely driven to the very end or point of the Pole of that World, but even to another Pole of another World, which joined close to it; so that the cold having a double strength at the conjunction of those two Poles, was insupportable: At last, the Boat still passing on, was forced into another World; for it is impossible to round this Worlds Globe from Pole to Pole, so as we do from East to West; because the Poles of the other World, joining to the Poles of this, do not allow any further passage to surround the World that way; but if any one arrives to either of these Poles, he is either forced to return, or to enter into another World: and lest you should scruple at it, and think, if it were thus, those that live at the Poles would either see two Suns at one time, or else they would never want the Sun's light for six months together, as it is commonly believed: You must know, that each of these Worlds having its own Sun to enlighten it, they move each one in their peculiar Circles; which motion is so just and exact, that neither can hinder or obstruct the other; for they do not exceed their Tropicks: and although they should meet, yet we in this World cannot so well perceive them, by reason of the brightness of our Sun, which being nearer to us, obstructs the splendor of the Sun of the other World, they being too far off to be discerned by our optick perception, except we use very good Telescopes; by which, skilful Astronomers have often observed two or three Suns at once.
No sooner was the Lady brought before the Emperor, but he conceived her to be some Goddess, and offered to worship her; which she refused, telling him, (for by that time she had pretty well learned their Language) that although she came out of another world
The rest of the Inhabitants of that World, were men of several different sorts, shapes, figures, dispositions, and humors, as I have already made mention, heretofore; some were Bear-men, some Worm-men, some Fish- or Mear-men, otherwise called Syrens; some Bird-men, some Fly-men, some Ant-men, some Geese-men, some Spider-men, some Lice-men, some Fox-men, some Ape-men, some Jack daw-men, some Magpie-men, some Parrot-men, some Satyrs, some Gyants, and many more, which I cannot all remember;
A different universe (or part of the universe):
Having thus finished their discourse of the Sun and Moon, the Empress desired to know what Stars there were besides? But they answer'd, that they could perceive in that World none other but Blazing Stars, and from thence it had the name that it was called the Blazing-World; and these Blazing-Stars, said they, were such solid, firm and shining bodies as the Sun and Moon, not of a Globular, but of several sorts of figures: some had tails; and some, other kinds of shapes.
At last, the Empress commanded them to go with their Telescopes to the very end of the Pole that was joined to the World she came from, and try whether they could perceive any Stars in it: which they did; and, being returned to her Majesty, reported that they had seen three Blazing-Stars appear there, one after another in a short time, whereof two were bright, and one dim; but they could not agree neither in this observation: for some said, It was but one Star which appeared at three several times, in several places; and others would have them to be three several Stars;
Wherefore I'le conferr with them, and enquire whether there be not another World, whereof you may be Empress as well as I am of this? No sooner had the Empress said this, but some Immaterial Spirits came to visit her, of whom she inquired, Whether there were but three Worlds in all, to wit, the Blazing World where she was in, the World which she came from, and the World where the Duchess lived? The Spirits answered, That there were more numerous Worlds then the Stars which appeared in these three mentioned Worlds.
Besides, said she, the Fire-stone will serve you instead of Light or Torches; for you know, that the World you are going into, is dark at nights (especially if there be no Moon-shine, or if the Moon be overshadowed by Clouds) and not so full of Blazing-Stars as this World is, which make as great a light in the absence of the Sun, as the Sun doth when it is present; for that World hath but little blinking Stars, which make more shadows then light, and are onely able to draw up Vapours from the Earth, but not to rarifie or clarifie them, or to convert them into serene air.
Thus after all things were made fit and ready, the Empress began her Journey; I cannot properly say, she set Sail, by reason in some Part, as in the passage between the two Worlds (which yet was but short) the Ships were drawn under water by the Fish-men with Golden Chains, so that they had no need of Sails there, nor of any other Arts, but onely to keep out water from entering into the Ships, and to give or make so much Air as would serve, for breath or respiration, those Land-Animals that were in the Ships;
In this story, there's much talk about Stars in the sky. Even the world is named Blazing-World because of the blaze of all the stars in the sky is so bright that the night isn't dark, like in our world. When they observe our world they see only 3 stars, but they can't even decide if it was 1 star seen thrice, or three stars seen once apiece.
If these worlds were actually physically joined at their poles, such as two spheres stacked atop another, then there should be more visible to the astronomers with their telescopes.
How this reads to me is that the two worlds are either:
- Joined at their poles by a space-time bridge, such as a Tesseract from A Wrinkle in Time
- Coexisting in the same "space" but on different planes
I lean towards the second explanation, because throughout the tale there is mention of Spirits and Souls. The Lady speaks with both of these immaterial beings (and has the Soul of the Duchess of Cavendish, the author, brought before herself). This indicates that the reality/plane that the Blazing-World exists on is different than that of ours, which has only material beings visible.