All World-Building is Modeling
First, I want to propose that all world-building is model building. When I say “model” you may think of something technical, like a climate model running on some super computer. But there are many kinds of model. They can indeed be mathematical, but can also be purely mental, drawn on paper, or made of clay. A model is really just a (theoretical) understanding of a system.
The systems of coinage used in the middle ages were more complex and more interesting than what's presented in most fantasy role-playing systems: if gold, silver and copper "pieces" with nice metric conversions seemed too neat to you, you were right; read on.
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.
What business do a man, an elf, and a dwarf have traipsing the countryside looking for quests? Where do they come from, and how do they fit into their society? Can they exist at all? I'll give a few answers to these questions below.
The stock fantasy world of role-playing is, we usually think, basically like medieval Europe. There are castles, swords, peasants; various set pieces. But what do these worlds of abundant cash, swords for hire, open-minded villagers, and "mage guilds" really have to do with the middle ages?