So your game world - Do you know its history? Probably. Probably have a good idea of how some of the kingdoms came to be and what might have happened to some of the kingdoms or empires that have fallen. That’s good!
Here’s the trap - If you learned history in a USA school, then your chances of legitimately getting the context of history are nearly 0%. I don’t know about other countries’ schools, so I won’t get into that, but since the American style is based on the Prussian model (where did you think “kindergarten” came from?), I’m guessing this is more than likely true beyond our borders. Any history teacher that had you memorize dates and names, but did not actually draw out time lines cheated you of understanding the context.
It’s really important to understand that during the “Dark Ages” Muslim scientists were making fantastic advances in mathematics and other sciences. In fact, it was Muslim map makers who made Columbus believe he could sail around the world to India. (Not their fault - Columbus was the idiot who got his measures wrong - think of it as reading feet when someone said meters, not a good analogy, but close enough.) I could go into a ton of other examples (barbarians vs. Rome and why, Chinese trade expeditions, etc.), but that is not what this post is about.
In your world, you figured out where and when some of the wars were, right? You know how the winners ended up, because they took over the land. But what about the losers? Did they all just live as subjugated people? Were there refugees who moved somewhere else changing the population of that place? Were/Are there pirates and other sea raiders who plunder and pillage the sea shores? Have they driven those coastal people inland? Was there a plague? a famine? a really crazy and sadistic king? What happened after all those people died?
Really important historical events have a ripple effect. People get pushed from place to place. Resources get used up. Ruins get left behind, and sometimes repopulated. If the rampaging horde pillages the center of the country, but is eventually put down by the heroic northern city state - What happens to the center of the country? For that matter, what happens to the place the horde came from? Now that they’re gone, their enemies will want to move in, so you might get a fierce civil war. Meanwhile, someone is going to move in and take the center now that it is basically abandoned. Also meanwhile, the heroic victors are now low on adult men due to casualties and low on steel because it all became weapons and armor. Do they allow foreigners to start marrying widows? Do they turn to bigamy? Do they turn their swords into plows? One big war and several ripple effects.
This is not just true of wars, but it is easiest to see there. The Black Death is likely the cause of the rise of the middle class and the end of the serf system (because labor was all of a sudden really valuable). I know we don’t like to talk about it, but slavery worked because the crops from this previously unknown and under populated land suddenly became worth huge amounts of money overseas. Thinking back to the horde example - During their civil war, would the losers be sold into slavery to the new folks in the center land because they needed the additional labor?
I’ve been working on Fletnern for well over 30 years and I still come up with ripples as to why certain things are what they are. Nothing makes me happier than when I look at some historic event and know that something else happened around the same time, and I can have one of the events actually cause the other. I love it when a plan comes together! Give it a try. If nothing else, you’ll write a lot more history than you have documented now.