Later, I started to think more deeply about what a world with demigod-power adventurers, reliable magic-users, and ruins from advanced ancient civilizations would actually look like. I eventually realized that non-medieval anachronisms were not only possible, but inevitable. There is no way that a world that operates on the rules presented in fantasy RPG rulebooks would look like medieval Europe, or any other specific historical setting.
Now I will use almost any minifig that shows up in the bulk brick lots I order from shopgoodwill.com. It helps that my games have a lot of content inspired by Spelljammer, Planescape and Eberron, but that is probably not necessary. A recent game had a lot of orcs clad in a motley assortment of various armor scraps, and nobody even noticed or cared that one of them was stormtrooper armor.
The rest of this post will not be about Lego, but will instead be an extended discussion about why no fantasy RPG world will ever look and feel like any historical period. Even if the technology level is the same, the shape of society will be radically different.
Military Technology Shapes Society
A fundamental lesson of history is that the structure of society depends on its military technology. The Middle Ages were the way they were because the most powerful force on the battlefield was the mounted knight in armor. Knights were the foundation of military power, which meant that they inevitably accumulated economic and social power as society grew to serve them and their needs. The medieval manor was a self-contained economic engine devoted almost entirely to the creation and maintenance of armored knights. Everything served them. The economy was devoted to giving them horses and armor and fortifications. The culture of the nobles focused on identifying and developing the traits that made men good knights.
Contrast this with ancient Rome or warring-states China. In those places, the most powerful force on the battlefield was a disciplined, professional infantry formation. As a result, their economy and society were organized around maintaining these infantry armies. Things were more centralized and standardized, with large economically integrated empires connected by good roads, and their culture valued discipline, scholarship, and bureaucratic competence much more than 'chivalric virtues' like bravery.
Also contrast the middle ages with what happened after firearms were invented. Before firearms, a mounted knight could defeat an arbitrary number of peasants on the battlefield. After firearms, the number of bodies on the battlefield became much more important. This shifted power away from the aristocracy and toward the people. A revolution by armed commoners became a much more serious threat, and governments responded by giving them more rights and/or inventing new technologies of social control like propaganda and nationalism.
Background: Military Supremacy of AdventurersThis section explains why powerful adventurers make traditional military forces obsolete; skip it if this fact is self-evident to you.
In the world of the fantasy RPG, the most powerful military force is a party of high-level heroes. In most game systems, a group of high-level adventurers with basic tactical skills can easily crush a nonmagical pre-modern army of almost any size. Even if they can't wipe out the opposing force in a single day, they can easily use their superior mobility to retreat, rest, and come back later to finish the job.
The superiority of the adventuring party is even greater when you consider the issues of deployment and force projection. A premodern army will only move 10 to 20 miles a day, and unless they are backed by an empire's worth of supplies and logistical competence, they will strip the land bare like a horde of locusts as they move. By contrast, an adventuring party can use flight, an airship, or magical mounts (or just a teleportation spell) to deploy to where they are needed very quickly and cheaply. This would make them an incredibly valuable military asset even if they were weaker than a large army in open combat.
Furthermore, the logistics of an adventuring party make them even more attractive and useful as a military asset. Unlike every other military force ever, a high-level party requires no supplies at all to function, not even food. (By high levels, any sensible party will have magical means of creating food and ammunition, and repairing their gear.) While they are more effective with a supply of healing potions and other consumable items, this is not at all essential for them to completely dominate other types of military forces. If they are loyal to you, or your cause and values, their incredible power comes with no support cost or logistical demands.
Additionally, the nature of the world makes access to a high-level party essential for basic survival. Even if you are a peaceful country with no intention of going to war or conquering anyone, and/or you have a mundane military sufficient to avoid being attacked or conquered by any of your neighbors, you absolutely must have the ability to contact an adventuring party and hire or persuade them to save you from demons, dragons, vampires and other existential threats that no army can stop.
What the World Will Look Like: Differences
As described above, high-level parties of heroes make conventional military forces almost useless. There is very little reason to drill armies or train knights, or to construct many of the things associated with them. Large standing military forces of any kind will simply not be a feature of society. There will be police, but no army.
There may or may not be any contemporary castle construction. Militarily, there is no point in building castles. Castle walls will not even slow down a mid-level adventuring party, or any other real threat like dragons or demons. However, if the world is infested with dumb wandering monsters, like beasts and zombies, then most towns will be fortified with walls and/or stockades to keep them out.
In a fantasy RPG world, the people with the most power will inevitably be the ones that control or influence the most high-level adventurers. All of the economic, social, political, and religious institutions of society will be devoted to the task of identifying, training, supporting, rewarding, and ensuring the loyalty of people who are or could be part of well-functioning high-level adventuring parties.
As long as adventurers exist at all, (most modules assume they are common enough that people are used to dealing with them) it does not matter how rare adventurers are. Their mere existence is sufficient to warp society around them, and the rarer they are, the more they will be treasured. Here are some specific social features of that world:
There will be as much gender equality as the nature of economic production allows. Female adventurers are just as powerful as male ones, and no society can afford to waste their potential power or do anything to make them upset with the current order. Even if NPCs have human sexual dimorphism, which means that the upper-body strength of male agricultural workers makes them more productive in plow-based agriculture systems, which will inevitably generate some economic inequality, there will be far more social equality than in medieval societies.
There will not be any witch burning. Anyone with magical potential and any ability to function in a team is far too valuable of an asset to damage, and every authority in the land will make this very clear.
Because adventurers can come from any social situation, political organizations will work very hard to ensure that all of their people will have some loyalty to them. There will be as much social and economic equality as the state can afford, and there will not be any groups of oppressed or marginalized people (in the PC races. Races that never or almost never create adventurers could easily end up abused and enslaved.)
Society will heavily encourage people to try their luck at adventuring, and will subsidize any activities that are likely to generate adventurers. If a collection of random village teenagers wants to go exploring the world, the authorities will supply them with training and equipment and aim them at a low-level questing ground. If they die, then the state only lost a few peasants, but if they come back with class levels, the state gained an incredibly valuable military asset.
Groups of low- or mid-level adventurers will be carefully nurtured and supported, especially if they are relatively cohesive. At minimum, there will be a heavily subsidized 'adventurer's guild' that gives them basic supplies and support and information about level-appropriate quests. Anyone who can tolerate the regimentation will be recruited into the military, given an income, rank, and status, and deployed to level-appropriate missions.
The most valuable economic asset is not land or gold or trade goods. It is high-level items. They are incredibly valuable military tools by themselves, and they are also one of the best ways of attracting, paying, and ensuring the loyalty of high-level adventurers. States will care relatively little about controlling land or gold mines or trade routes, but they will care a lot about controlling sources of powerful items, or information about their whereabouts.
If it is possible to create magical items, the entire economy (after the labor and resources required for subsistence) will be devoted to doing so.
Conquering and holding territory is very difficult. Adventuring parties can easily destroy armies of conquest, but it is very difficult for them to hold territory and collect taxes. People are unlikely to fear conquest and invasion (although they might fear evil adventurers demanding exorbitant amounts of protection money, or simply looting things).
Collecting taxes is much less necessary, and much harder to justify. As long as the local leaders have a connection to a reliable adventuring party, there is basically no need to support or train an army.
Similarities to Premodern Societies
However, despite these differences, there will be ways that the world resembles medieval society:
It is likely to be a world of smaller manors and city-states rather than empires or cohesive nations.
The machinery of state is likely to be weak, there will be very little bureaucracy or regimentation.
Travel will be very difficult for ordinary people. Because adventurers do not need roads, they will not be built. This will make the world very fragmented, and people very provincial.
Because of the travel difficulties, merchants will be rare, and most social units will be in a subsistence autarky.
Knowledge and scholarship will be rare, aside from things that make adventurers more powerful. Nobody has much incentive or ability to research engineering or agriculture, but they do have an incentive to research magic spells, combat techniques, and locations of ancient items of power.