How can a part of a continent (super or other) remain unexplored by the apparently civilized majority? The undiscovered part must be habitable but inaccessible, by land or sea, so that the rest of the world is completely in the dark about their methods of war, culture and the flora and fauna. The technologies used by the rest of the world for exploration are no more than ships and scouts.
People do discover it. They just never report back.
It's one thing to sail all the way to a new continent. It's quite another to land there, chart the territory, and then sail back home again to tell people what you've found. Perhaps the coastal waters are exceedingly treacherous and the ships keep getting dashed to pieces. Perhaps that continent is inhabited by deadly predators that kill anyone who lands there. Perhaps the locals view the explorers as "invaders" or "demons" or whatever, and kill them on sight. Alternately, maybe it's such a wonderful place (at least on the surface) that none of the explorers ever want to go back home.
Whatever the reason, nobody has ever returned from that place to be able to tell other people that it exists. From a world-building perspective, I like this idea better than just "it's really remote", because it opens up so many other possibilities. For example, it could have spawned in-universe myths or legends about this region that people sail to, never to return, rather like the Bermuda Triangle.
Looking at Europe, Siberia, and East Asia might be a good precedent or model.
- Introduce an inhospitable mountain chain or two.
- After those mountains there will be swamps, insects, more swamps, taiga, and swamps. People live there, but they don't have much to trade beyond the occasional pelt, gold nugget, or piece of amber.
- Living in those swamps requires the right survival skills -- how to find potable water, how to prevent food and other supplies from rotting, etc.
Now dial this up. The mountains are not like the Urals, they're like the Himalayas. The swamps are not like the West Siberian Plain, they're more like a jungle.
This will not prevent all travel, but it will turn the land at the other end of the continent into a mythical "here there be dragons."
It is far away
Quite simply, in both Europe and China of 1200 AD there was essentially zero knowledge of the other civilization. The Chinese didn't know about Europeans or European crops/trees/animals, and the Europeans didn't know about the Chinese and their crops/trees/animals. So while there was some exchange of crops and technology though nomadic middlemen, in general this fits the bill, exactly.
If you would like to make them even more isolated, you can remove the middle men. If the Indian subcontinent dissapeared, then Europe and China would know even less of each other. If you removed the Amu Darya and Syr Darya rivers and the city states of central Asia, then even technology and crops might mot have passed from one to the other.
There are some very fun answers here already. One even got accepted as the correct answer by OP. Seem to be forgetting the major one, though.
Italy is lovely. Beautiful summers, lovely winters, enough farmland to sustain urban settlements and enough coastline to host maritime republics. By around 1300, you had portolan charts starting to get so detailed and accurate that they look like someone carefully tracing a GPS plot.
Sure, you can have swamps or desert but if there's anything useful in them people will have a line going across directly to it and know all about it. Meanwhile, just make things a little chilly and it takes until 1430 to get this map of Scandinavia
and it's not until 1540 that you get something as still-inaccurate as this
We were still debating whether the Norse had made it to Greenland and North America into the 20th century and the two largest countries in the modern world are barren arctic wastelands because no one wants to go anywhere really cold or talk about it once they get back. It's gotten a little better recently with thermal fleeces, industrial-strength insulation, &c. but if you want real-world reasons for people not to explore or know much about a place, make it arctic.
There are many examples of this in our history. Taking medieval times to be 5th-15th century we only have to look at the deluge of new discoveries which came after that to see that there was much which lay out of our reach before that.
There are many similarities between these new discoveries but in general they are separated by something too costly to traverse, an ocean, a desert or sometimes they're just a small island far from anything large and the probability of coming across it is just too small.
To build your world I would suggest incorporating any one of these into your world to separate your one civilisation from the rest.
Fear. It's a powerful motivator and a great tool to control the masses.
Before maps and navigation aids, it was fear that stopped people in ships from sailing beyond the horizon away from the shore. It was the fear of falling off the edge of the planet that kept them hugging the coasts.
Fear of spirits, demons and gods can keep people off a section of land. Fear of a people, imagined or not, that attack everyone that enters the land can keep that land unexplored. Fear that there are no riches in a land, only an endless landscape of fire and brimstone. That place? That place we don't go? In that place, there be dragons.
Africa proved very difficult for Europeans to explore for a variety of reasons. The presence of the Sahara Desert across the northern expanse discouraged exploration by land - the harsh environment presented more danger than most were willing to brave, considering there was a good chance you wouldn't find anything except miles of featureless, inhospitable sand. Sailing down the coast of Africa was dangerous because returning was very difficult - the shape of the continent meant that ships that rounded the edge of western Africa would be constantly going against the wind on the return journey, and until ships were developed that could tack into the wind there was a very real fear that any ship attempting the journey would be unable to return. The political situation of the time made trying to travel through Arabia difficult for Europeans. And as others have pointed out, those who managed to make the journey would also have to avoid disease, unfamiliar predators, and other dangers long enough to return.
I propose your civilized folks could discover these places, if they would get off their asses and go. Or hire someone else to. You can depict a degenerate, decadent and pampered civilization. They have everything they want and have no particular desire to find anything more or different, especially if it involves discomfort or effort.
Someone has to do the work, though. If your upper class is as described and the lower class is thoroughly beat down, illiterate serfs, there is no middle group of scrappers and self-made men that can give rise to a Columbus or Magellan. That leaves the military and is is no stretch to posit a hidebound and tradition-minded military uninterested in anything new.
For a long time nobody who went there ever came back. Then someone did come back. Dying. And started a plague that killed half the town they came back to. Nobody goes there any more. People suspected of returning from illegal trips are shot on sight, and wide rings burned around their bodies.
Two more types of barrier that haven't been mentioned:
Super murderous inhabitants (plant or animal) surrounding the area. A forest full of poisonous plants, active predators and maybe even violent natives could easily deter exploration. This was basically the status of parts of the Amazon until quite recently.
Surround Fields of lava. Perhaps such an event could surround a habitable region.
Expanding on the various people that mentioned plague. It's easy to say "everyone that went there died" but that's actually not a useful deterrent because people are dumb and keep trying anyway. Here's a better idea: let's say you have an area that people have visited but got sick but people got better if they left the area. That's easily tested and consistent. People go, they all get sick, they leave, they get better. Send some prisoners and a rotating set of guards, everyone gets sick, rotating guards survive, prisoners don't. Rinse and repeat in a few areas and find it's the same.
Works best when you have a limited amount of access or an isolated continent, but you can work it on a single continent by environment trickery. Think mosquitos but a bit less picky and a lot more lethal... pathogen spread by something that is very common, is about the size of a mite, but can only survive in that climate. Have it not be too fussy about hosts and the surviving fauna are immune. There's enough variables to play with to get a suitable deterrent.
Rocks. No, really, have a large chunk of continental crust made up of a ridiculously toxic material. Let's say that some weird event during the formation of the planet resulted in a wide area that was so contaminated with radioactive isotope bearing rocks that anyone spending any amount of time in that area would suffer radiation poisoning. We know from experience that plants and even some lower complexity species can survive in these environments just fine but a lot of larger creatures struggle or just die off. You can write it off as a stellar fragment, an earlier civilisation having a nuclear war, or something silly like a narrow cross section gamma ray burst early in formation.
Tectonics. Have an area that suffered so badly from volcanic activity and earthquakes that previous generations decided it wasn't worth spending time there... maybe the knowledge that the area was bad was passed down but the reason and any real exploration results have been lost.
The first and third are adaptions of ideas from Pern. Fun series with a similar obstacle.
The people could have a religion which says that it is forbidden to walk in this lands, maybe because they are the holy land of their god and no one should ever walk there. This would be a good option if this area is rich on ressources.
Or the complete opposite. It's an harsh land, so this is the land where the devils and demons life, and everyone which goes there will die and worship demons.
Both options give one good thing, everyone which was there and tells about it is a heretic and traitor and will be executed. And no one will believe what they say, because they have violated the holy laws of their religion.
Perhaps a labor shortage has made an expedition not worth the money.
The great plague killed 19 out of every 20 people. They were referred to as "The lucky ones". Shipbuilders were hit the hardest. We lost most of our trade fleet because of foreign powers burning infected vessels. It has been over 150 years. Our population has not recovered. We'd like to send a ship to investigate that continent we found out about, but we can't spare the people to make a crew, spare the shipbuilders to make a vessel that's capable of such a long voyage, or spare the people to prepare for such a long voyage. It would take several people we can't afford just to make a 3 year supply of fishnets!
A technological society doesn't want to be discovered
While the main society is inching into their Renaissance, embracing technology and thinking they are the pinnacle of civilization....
... in fact, the people on that faraway continent are centuries ahead of them - with radar, thermal imaging, satellites and maritime patrol aircraft. And the Aircraft Society uses these capabilities to intercept 100% of Medieval Society explorers. The medieval explorers have no ability to hide, and it wouldn't even occur to them to try.
Who knows, perhaps religion has everything to do with it. It doesn't need to be Aircraft Society's.
How they deal with explorers
Aircraft Society either integrates explorers into their society, interns, imprisons, or kills them, or perhaps some combination thereof depending on circumstances. Integration is no small motivation to an explorer's mind, it would be like being invited into Heaven itself.
They might operate an intelligence network within Medieval Society, either observing: collecting advance notice of most expeditions, perhaps pulling out explorers' families when that is negotiated. Or actively tampering with their development, e.g. buttressing religion, amplifying superstition, inciting persecution of technologists, and trying to keep them in the Middle Ages. Perhaps Aircraft Society is in the throes of its own enlightenment, changing how they manipulate Medieval Society.
Their motivation might be "Prime Directive" sort of non-interference. More likely it is self-serving.
Perhaps they see amazing art and culture arising from Medieval Society, and they don't want to corrupt it the way modern technology has corrupted so many formerly isolated societies.
Perhaps they feel Medieval Society is just not culturally ready for an influx of unearned technology and would horribly mishandle it to their own destruction.
Perhaps the Societies' differences are irreconcilable, and they are trying to forestall as long as possible what will definitely happen when Medieval Society discovers what uranium does.
If the native Americans had killed Colombus on the spot, their civilization may live even today.
Also Montezuma could have killed Cortez many times, he didn't do until it was too late.
Antarctica wasn't found for a very long time, simply because it was cold and it was too far away. Thus, weather can be another reason.
If North America had been a small island, like New Zealand, around where is California now, Colombus probably would never have found it. It may have taken centuries until iron steam ships were to find it.
er... are you aware that during middle age, in fact, Europe and Asia were completely unaware of the existence of America and Oceania? That is the exact scenario you are describing. And also most of Africa was unexplored (from the European point of view).
As far as we're concerned, in a slightly alternative world, in the Americas there could have been civilization at least as advanced as in Europe, and the two civilizations would have had no contact with each other.
So, in a middle age scenario, that is very likely to happen. Actually, it has in fact happened. Having an ocean in the middle, is a perfectly valid reason.