Fantasy RPGs often assume a pseudo-medieval setting. It’s not a true fit, but a basic understanding of medieval demographics can really help make a setting more credible. At the same time, having regions that seem credible make the places that are different seem more incredible. Having a clear baseline for ‘normal population’ is useful both ways.
- Medieval Demographics Online is a useful tool at The Welsh Piper that can walk you through the processes described in Low-Fantasy Populations (a pretty epic bit of research itself).
- Medieval Demographics Made Easy was written by S John Ross is just as it says on the label — a simple treatment and description of medieval demographics. However, while it is simple, it isn’t simplistic, and is probably adequate for most gaming purposes.