In 2015 I ran a Majestic Wilderlands campaign using D&D 5th edition. One of players was +Douglas Cole who blog frequently on Gaming Ballistic. He also kept a journal of his time in the campaign. Given the interest over in campaigns that are Medieval Authentic I figure it would help to show an example of actual play in a campaign where a lot of the material is drawn on medieval history.
One technique I use is re-skin history. I will take one or more incidents, change names, and make the background fit. Earth's history is incredibly diverse and easy to find stuff to adapt to a campaign if you are well-versed in history.
For example, Saladar, King of the Grand Kingdom (from Blackmarsh, Points of Light) just died and his only child surviving to adulthood is the Duchess Aleia. Alias was married Geran, Duke of Powin, part of the neighboring Kingdom of Gwyneed. However the Grand Kingdom is only two generations old. The seven realms having been united by King Aldric the Bold, Saladar's father, 50 years ago. When Aldric died, Saladar's older brother, Aldric the Red (II) took the throne but died in a hunting accident a few years later. As he had no child Saladar was crowned king. Unfortunately Saladar's eldest, Prince Edwin, died while patrolling the eastern borderlands leaving only Aleia as his heir.
However Barons did not like the idea of a Queen being in charge. Their hold over their estates throughout the seven realms was tenous and they desired a strong warrior king like King Aldric the Bold. Palanon was the grandson of King Aldric the Bold, however his mother was King Aldric's daughter so he wasn't considered as a candidate for the kingship until King Saladar died without a male heir. Palanon's supporters moved quickly as it would take two month for the Duchess Aleia to arrive at the capital of the Grand Kingdom. The royal treasury was seized and Palanon was proclaimed King just as the Duchess crossed the borders a few week later.
The Duchess Aleia was known for her fiery temper and wasn't about to give up. She issued a call for loyal supporters to gather under her banner. And with that call the Grand Kingdom was plunged in the era of the Chaos.
Now if you don't recognize any of this, this is a thinly veiled re-skin of the Anarchy period of England. Saladar is a King Henry I who lost his son in the shipwreck of the White Ship, leaving only his daughter the Empress Matilda. Palanon is Stephen the grandson of King William the Conqueror by his daughter Adela of Normandy.
But it not enough
OK so you made the above background and re-skinned it nicely. Right now it is useless vanity piece you wrote for your enjoyment. The only thing that makes it matter is how it defines the behavior of the NPCs that the PCs will encounter.
If you read medieval history a lot what drives things is the social webs that surround those in power. In one sense the Anarchy was two biker (or horse?) gangs (Matlida, and Stephen) fighting over turf with various allied gangs coming and going for their own reasons. So the next step one need to take is to define who does the Duchess Aleia know, do they support her (or not), and why. The same for Palanon. For myself I try to keep to the rule of the half-dozen. At each level I only focus on a half-dozen npcs. That way I can keep things in my head. The actual number should be whatever you are comfortable with keeping in your head. Some can to more and some need to do less.
For this type of background what will impact the players the most are retinue of the lowest ranks of nobles with land and power. Those who follow Barons, Sheriffs and the like. So sketch out Palanon, and Aleia, half dozen of their main supporters, and then of them pick a handful who impact the area the PCs are starting in. Then define their retinues and the NPCs that the players are most likely to encounters. Chancellors, Bailiffs, Captains of the Guard, etc. The retinue depends on their lord for privilege and survival so whatever whom the lord is backing they will back.
Finally this is the average, there will be exceptions and variants. But you have to have a norm. So keep a rough count of how often you make an major exception to the rule and if it exceed half of what your are detailing then quit writing up exception and go with mild quirks to add color. For example you defined a regions that has two Barons and five knights. No more than a single Baron and two the knight should vary greatly from the feudal norm in some way. The rest will have quirks to add color.