So you've laced your boots, strapped on your scabbard, and set your jaw. You're ready for some adventure. Where though will you go to find it? Where are the lonely places, where monsters, treasure and glory can be claimed? In this quest, you face some problems.
There is no wilderness
Through most of the middle ages, western Europe was essentially fully settled. There were protected forests and some places too harsh to live, but humans had spread through the entire region. There really weren't huge tracts of wilderness separating settlements: the fields of one village generally ran into those of its neighbors. Indeed, this is why Europeans were so hungry for wood from North America: they had used up their own long ago.
Within the agricultural landscape, there were some relatively unmanaged places. There were hedgerows of trees and dense bushes separating fields; many remaining untouched for hundreds of years. There are meadows and agriculturally marginal pasture-lands, often in places of higher elevation. Forests existed but were not, as we tend to think today, wooded places: rather the term was a legal one and described protected spaces, usually for hunting; often they were quite open--the better for deer.
Things look bleak for a would-be explorer then. But there were exceptions to continuous settlement. Mountainous areas, especially in the north, could be difficult to live on and used only seasonally for grazing. More grimly, disaster could severely depopulate an area: plague might kill two-thirds of a town or more. Such a loss, or famine or war, could force total abandonment, at least for a time. Even if then, the lands are hardly virgin and will be resettled as soon as conditions allow.
- Abandon the idea of wilderness. Adventurers do jobs in cities and towns, and between villages.
- Envision a thorough societal collapse in the near past, such that towns and fields have been abandoned for at least a generation. If monsters in your world are quick to move into such places, it could greatly delay resettlement. Perhaps aiding this process is exactly the aim of the adventurers (or their patron).
- Imagine a newly-colonized land. What if an equivalent of the Americas had been found by Europeans 300-years earlier? (What if it really had been unpopulated?)
There are no dungeons
Adventurers need not just wilderness--in which to have encounters most random--but also destinations. Cities and villages, even semi-isolated castles, are no great difficulty for the historically-minded gamer. But what of these profligate dungeons, filled to the brim with pokable evil?
Let's begin with simpler aboveground ruins. One difficulty arises immediately: most ruins, having once been operating buildings, are naturally near other buildings; near civilization. This means that they're likely to have been picked clean of anything valuable. In fact, stone itself was a valuable building supply and would be recycled into new construction if at all convenient; and people were not shy about taking up residence in a half-ruined church or, as happened in Rome, a colosseum. In other words, adventurers are unlikely to be the only ones interested in ruins.
Ruins further away from towns might survive for a while, for instance, a remote border fort, which provides its own reason for existing. But if an outpost remains on a border, why should a king not try to re-use it quickly? Probably we must invoke one of the solutions used above, to create wilderness more generally, so it can be abandoned for a long time. Still, one wonders how many such forts are really left hidden over the long term, waiting just for the protagonists.
The nature of dungeons is a full topic in itself. But we can note for now that multi-storied below-ground complexes of the kind often imagined in fantasy are startlingly absent from the archaeological record of Europe. The technology to mine at all deeply was not well developed until the later middle ages (more to do with air and waiter than digging per se), though some areas lend themselves to it, geologically, and the Romans did have some extensive underground works.
Still, simple mines or storage vaults are a far cry from elaborate dungeons. Even given the tools, who would build such a thing and for what purpose? Given that purpose -- and presuming that it was left untouched -- would valuable jewels really have been secreted there?
- Abandon the idea of dungeons altogether.
- Replace "dungeons" with simple "ruins." Imagine great social upheavals presently or in the very recent past, so many of these ruins are fresh and can be looted, probably for wall sconces more than loose items.
- With he help of magic, far more expansive underground structures are available. Because of constant warfare (between mages?) these are regularly built and quickly destroyed or abandoned, thus preserving hope of loot.
The latter two solutions obviously interact with earlier points about the commonness and acceptability of adventuring. Abundant ruins with valuables could make adventuring a common activity; but if adventurers are too common, because adventuring is too easy, they will plunder the ruins into nonexistence. The upshot really is that for adventuring in wilderness and dark catacombs to make sense, the world must have seen major upheavals; or still be seeing them.