Joining a kingdom
When you get to a level 4 Stronghold, you will be able to join a kingdom. Also, there will be a quest triggered called Banner which will lead you to join a kingdom. When you are level 4 Stronghold, you can create your own kingdom, or join one.
You can find the right kingdom for you through the Search feature, or you can see the kingdoms in the leaderboard. When you click on the name of a kingdom, an Information page will pop up where you can see the info about that kingdom.
Below you a kingdom you will either find Open or By Request, which is their joining status.
If the status of a kingdom is open, they will allow everyone to join, and if it is By Request, you will send a request to the kingdom and then they will decide whether or not you can join.
Leaving a kingdom
That was for joining a kingdom, but how about leaving one? How do you do that? Well, there are many ways to leave a kingdom in Rival kingdoms. The way probably no one wants to leave is by getting kicked out of the kingdom. You can also select the leave command you can find in the kingdom Information page. You can request to join a different kingdom and leave in that way. Resignation from the position of a king means you’re leaving the kingdom. Also, if you are the last one in your kingdom, if you leave, nobody will be able to join again, not even you.