Here's a way to incorporate the Caste System into your game, ruthlessly stolen from the brilliant Small but Vicious Dog and tweaked to add some heft to Charisma. There are 8 ranks in the Caste System:

8. Queens, Kings, and Heirs. Anyone with a right to the throne.

7. Lords, Barons and Titled Gentry; the highest rank you can possibly be without being born into royalty. Own all the land.
6. Nobles, Aristocrats and Knights. Oversee the land for the lords.
5. Shopkeepers, wealthy merchants - anyone rich who had to work for their money.
4. Freemen. Commoners who can choose where they go.
3. Villeins: Peasants, commoners, laborers. The vast majority, working the land as slaves to a Lord. They are forbidden from leaving their lord's land.
2.  Lepers, beggars and guttersnipes. Loathed, but tolerated. Will not be allowed in most places.
1. Witches, devil-spawn, and mutants. Will be killed or chased from any major city or town if seen.

A PC is normally Rank 4, a Freemen; low class peasants who can still choose to go where they like.
Your Charisma modifier effects your rank. +3 elevates you to a Rank 7 Lord, -3 makes you an ostracized freak.  You also get + or - one dice worth of starting money per point of charisma mod. You gain rank naturally every three levels or so, but you can also move up and down ranks through your actions.

Pulling rank

The social demands of caste-system etiquette are subtle and complex. You can fuck with anyone below you.

One rank below you: You can ignore anything they say. They must give a good reason for disobeying any orders, or you're allowed to inflict any kind of non-painful punishment on them.
Two ranks below you: They must follow your orders. You can inflict any kind of non-lethal punishment on them for disobeying..
Three ranks or more below you: You can kill them without any legal ramifications or comment. They must follow any order, to death and beyond.

The same applies in reverse; don't fuck with anyone above you, or they could start throwing out punishments. The system is only enforced socially, obviously. Deep in the wilderness, outside of society, it's easy for the underlings to realize that you're just a weak man in a funny hat.

Unless you go a step further and use;

Divine Right

You aren't just a lord because your dad was a lord; you're a lord because God put you there. The medieval point of view is literally true in all aspects: The caste system is enforced by the universe, and nobles actually are better than peasants in every way.

When fighting someone of a different rank, find the difference between your ranks. Whoever's the highest rank gets +the difference to every relevant roll: To-hit, saves, damage, grapple checks, everything. The lowest rank gets -the difference to every roll.

So, if Graunch the Leper tried to take a bite out of King George, Graunch would be rolling at -6 to everything, while the King would get +6 to everything. This doesn't apply to anyone who doesn't care about the caste system.

A statement of intent: I'm not using these rules to stop my PC's fucking with high-level NPC's. I assume they'll still be constantly toppling governments like always. What I'm going for here is a way to give your Charisma score some serious social weight without forcing people to make Cha checks instead of roleplaying. 

With these rules, a high Cha makes you a force to be reckoned with in the city, and a low Cha could make you flee from any nobles you see. It even has hireling implications: You can order anyone three ranks below you to come on an adventure, whether they like it or not. Lepers or Peasants will be terrible at fighting, obviously: You'll still need to shell out coin for the good fighters. But being able to order up a mob on demand is pretty neat.