Whenever we look at a particular time in history, we have a tendency to assume that there was a continuous period that surrounded that time; a sort of cleared milieu in which things remained the same. This is a telescoping effect: the nearer the time was to us, the more gradient we see in individual years. For example, most of the 20th Century is typified as being a series of decades, each decade bearing a particular zeitgeist. The fifties were Leave it to Beaver, the sixties were Hippies and Free Love, etc. Of course, being so close to these periods, we know that they were really much more granular. The roots of each decade spring from the decade before, and rather than being discreet units they form a continuum of time.
What does that have to do with roleplaying? Well, it is an unfortunate tendency, partially due to the lack of sources and partially because of mid-late 19th century romanticisation of the past, for us to assume that certain periods were vast swathes of uninterrupted time. Imagine Rome in the 2nd century. Now imagine Rome in the 3rd. One hundred years separate those two time periods, but most people would be hard pressed to say how they differed. The so-called "long dark ages" have fallen prey to a similar problem in modern perception. The middle ages, at least in popular culture, tends to be divided into a series of very long periods, each of which has its own appropriate look and feel. The dark ages have vikings and mud, the Crusades have mail and helmets and the first fortresses, and the 14th century has the plague.
This is a problem that can only be defeated by adequate knowledge of the subject. The middle ages, much like modern life, were not a stultified mass of periodized aesthetics. Those with the eyes to see and the time to try will discover that each decade of the middle ages beginning somewhere around 450 CE and ending in the 15th century (for here Renaissance scholars will claim precedence) was vastly different. Every few years presented changing styles, scholarly opinions, even territorial guidelines. Kingdoms could be won and lost in the space of a few years. Charlemagne obliterated an entire nation of pagans every summer, chopping down their idols and firing their temples.
It is HARD to keep track of all these changes in history. It is even HARDER to keep track of them in a fantasy history. The worst thing about time is that it proves all things mutable, and in the middle ages there were few things more mutable than political fortunes. The great land-owning baron or count might tomorrow be dead of typhoid, or a hunting accident. A king, who's personal relations with his counts kept an entire kingdom from disintegrating, might fall from his horse and drown, causing five or ten smaller kingdoms to emerge from his once-unified whole. These radical changes were not rare.
It's easy for us to imagine a setting and then let it become stagnant. This kingdom goes here, that one goes there. In the case of long-lasting empires, borders may shift little (they are still shifting in huge amounts, actually, but the core regions of either empire probably remain consistent, thus providing the illusion of stability) but kingdoms based upon feudal relationships (even though that word is very out of vogue, I'm using it here to denote something apart from modern statehood) would be very subject to the whims of the world.
Take a look at your setting and consider what can change, and how fast it can change. The true hallmark of history is the mutability of all things. I know, for one, that I have a strong sense of nostalgia for my settings which leads me to lament when anything shifts; borders, populations, important NPCs... but shift they must, and the pain of losing that old status quo is really the pain of life. That's how you know it's working.