My advice is that you give all the Kingdoms and characters living there a full story and lives.
But do not drag the readers with all the history, but let it all reflect on how they react and lives.
Show, don't tell.
Example, a Viking meeting a Samurai.
You don't have to tell the history, but if you as the creator know their history you know how to treat them and how they would talk and fight.
Another great example is Boba Fett from Star Wars. He is only on screen for seconds, but you can feel that there is a huge story behind him