What are the (dis)advantages of having kings as vassals?

I'm newly the Wendish Emperor, as well as King of Poland and King of Lithuania. I also have enough of Pomerania to create that kingdom whenever I want.

Because I still have gavelkind succession (Poland's starting Crown Laws suck and I haven't had enough rulers yet to get high enough Crown Authority to change to Primogeniture), I'm contemplating giving away the King of Lithuania title and maybe creating and giving away the King of Pomerania title, so that I can choose how the Realm is divided up instead of having bits and pieces randomly redistributed on succession. (Right now a Polish county and the Kingdom of Lithuania would go to my second son, which would mess with the borders of the two kingdoms in ways I don't like.) I figure I can always remarry into those titles later anyway.

I've never played an Emperor though, so I'm unfamiliar with managing kings as vassals. I do already know that my Lithuanian dukes want the Kingdom and that has a hefty opinion penalty, but I don't know what sort of ambitions vassal kings have that might be worse. I've heard on various CKII wikis that kings as vassals are naturally "uppity", but I haven't seen any solid discussion of the mechanics that make them so.

I'm still on the fence about creating the Kingdom of Pomerania because I don't know enough to make an informed choice about structuring my Realm, and so long as I don't create the title my Pomeranian dukes and counts are happy as Baltic clams.

What are the advantages and disadvantages of giving away the King titles? Is it better to be emperor to kings or to dukes?

Considerations specific to kings/kingdoms as vassals

A kingdom with an active title will not de jure drift into your empire unless the kingdom's de jure empire title exists. (May have changed in patch 1.10)

Kingdoms have their own crown laws (crown authority, investiture), even when part of an empire (I am not completely positive, especially with patch 1.10, but that is what I have observed)

You cannot press de jure kingdom claims for your king vassals. The king can press his own de jure claims. You can press de jure ducal claims for your vassals, but not your vassals vassals.

Prestige. For a big enough kingdom (enough duchies) you would get more prestige from duke vassals than a king vassal.

Considerations similar to duke vassals, but amplified

Kings will want all territory in their de jure kingdom which will be bigger than a duchy.

With better marriage prospects, kings may obtain more powerful allies.

The increased power of kings makes them more capable to

• Rebel
• Conquer foreign territory
• Conquer other vassals in your kingdom

I have a Byzantium game that I began in 768; and reformed as the Roman empire in 973. Now, late game, it encompasses all of the Arabian Empire, Persia, Italia, Hispania, the Kingdoms of Mauritania, Alger, Walachia, Bavaria, Germany, England and Aquitaine. This would have been impossible to achieve and impossible to maintain across heirs without the careful use of vassal kingdoms.

Judiciously utilizing kingdom vice-royalties once your empire goes over the vassal limit the first time helps. Making sure your loyal (happy) dukes hold their maximum 2 duchies does too, but by the time you've two empires worth of duchies it doesn't cut it.

By the time you get there, if you've been handing out counties primarily and duchies exclusively to family members, your dynasty should be big enough to hand out kingdoms.

Rough guidelines for successful vassal kingdoms

1. Hand them out to only the best and brightest of your kinsman, who are not title claimants, and already like you. Now, they'll love you for the rest of your life. It also helps your dynasty score.

2. Give out kingdoms on the edge of your empire, vassal kings are far more powerful than dukes, they will conquer territory and they are far more likely to turn their eyes outward and be kind to their vassals and your other vassals if they are bordered by enemies (or targets).

3. Make sure you keep your piety and gold high enough to snap up and titles from their conquering before they can. It gives you prestige and the option to make other vassals or courtiers happy. Then if you want to, or need to, grant it to them yourself.

4. If a vassal becomes a problem or hates your heir, on your death bed, revoke it!.

5. When in doubt, test-drive with a vice royalty, the short-term hit to opinion is nothing compared to putting the wrong king or branch of the family in play long-term.

6. Don't raise a king who doesn't already have a male heir of your dynasty, Danger! Will Robinson, ensuring (as much as possible) your dynasty keeps that seat is always best and you can go on family focus if you have no other way to get them happy in a tight spot.

7. Don't ever have more vassal kings than you need to. Once you start raising kings up, make sure you run your vassal limit at the redline all the time, and hold as many kingships yourself as you can.

8. If your heir likes/loves you (60 or higher likes/loves you) give him two, it'll build his prestige and give him a chance to get his martial and stewardship up before you take over and makes a smoother transition, since he has a longer reign with a bunch of vassals already. if he likes you ok give him one see what happens, and if all's well after 10 years give him another. If he doesn't like you wait until you're older (50+) then give him one.

9. Always, if you have it, give the candidate a gift BEFORE you grant the kingdom it is the last affordable gift you'll give them.

It's a tradeoff. If you pick kings you will have less vassals and so you spend less money and time (and painful management) on bribing them. But civil wars will be bigger.

Having said that, you should not want to have king vassals. My experience is that with even a very large empire encompassing most of western Europe, even a single large king (such as France) joining a faction will push it to the point where the AI will issue an ultimatum (the point is around 35% - 40%). Then you will have a painful civil war. I have had kings join a faction (usually Independence) even with +100 opinion.

There are some considerations specific to Byzantines. Firstly, dukes can have their titles revoked without justification. Secondly, kings will have their own autocephalous patriarchs which means no excommunications (should you need to imprison them).

The primary advantage to being an emperor to Kings is having fewer direct vassals to deal with. That means fewer people to keep happy.

The primary disadvantage is that those vassals are commensurately more powerful. Should one of them rebel, you now have their entire kingdom to deal with in open rebellion.

I've never personally had control of an actual empire, even though I've ruled multiple kingdoms, so I'm not sure of anything more specific. The above is drawn from the guidelines about what's good and bad about having duke-level vassals as a king.