Wikipedia is the DMs Friend: Medieval Demographics

Most (A)D&D worlds are generally sparse in population, with but a single village in an entire 30-mile hex, if that. Many find that sort of emptiness unsatisfying from an historical perspective, and seek to beef up their worlds' population accordingly.

Many are probably already familiar with S. John Ross's excellent Medieval Demographics Made Easy page. In it, he crunches a bunch of numbers and comes up with likely numbers of people, occupations, population densities, and so forth. If you haven't read it, it's worth the time.

There's also an excellent and informative Wikipedia article on the subject of Medieval demographics. While it is not (obviously) focused on the sorts of data that would be most of use for RPG world-building, it does have a broader overall picture, stressing that demographics in Medieval Europe went through several booms and crashes over the centuries.

England in 1335 or 1365? It matters...

One of the interesting points I see there that would be suitable for the implied D&D setting is the High Middle Ages, where we see the "great clearances" of woodland and the expansion of agriculture. That would be a ripe time for isolated settlements in the wilderness, whose inhabitants are in the process of clearing away woodlands and badlands for farming, and in the process uncovering all sorts of long-forgotten stuff and disturbing all sorts of Things Best Left Undisturbed.

The period immediately following the Black Death would be an interesting RPG setting as well. Whole regions would be depopulated as people reorganized themselves, leaving behind towns and villages quickly falling into ruin, and into which long-displaced humanoids and monsters might creep to retake the lands taken from them so long ago. There might be lots of things out there in the newly-encroaching wilderness worth recovering, which gives a ready-made excuse for adventuring.

It should be remembered that Tolkien's Middle Earth similarly suffered a Great Plague in the Second Age. That's one reason you see all those old ruins scattered around everywhere in the films.

Think not just about what the demographics are in your campaign, but what the trend is. Are people expanding, and wildernesses therefore shrinking? Or is the reverse happening? Adventure is found in times of change, after all, and the type of change will shape the type of adventure to be had.

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Also, please don't forget the Adventures Dark and Deep Bestiary Kickstarter going on right now! 900 monsters, suitable for most OSR-type games, all under one cover. Can you help get us to having an illustration for each and every one?