Check the faq...
2. What is the purpose of a Kingdom and why do I want one? Kingdoms are smaller versions of the Campaign worlds – not just social lobbies. You are the Monarch of this world! You can explore, craft, build structures and even fight monsters… but a Kingdom is limited in terms of difficulty and power.
All accounts are granted a kingdom at the beginning of the game, but using it is not required (…beyond maybe a tutorial? We haven’t decided yet).
We fully expect that some players will prefer to spend the vast majority of their time participating in the Campaign Worlds, and we intend to let them do so without