You gotta pvp other users for it, you can do this until you reach 40 territory.

Go to any den of a king that has less stats than yourself & scroll down til you reach "Attack This Lion" & select territory. If the territory option is not available than you need to find another king to attack.

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The wiki has a section explaining "PvP" and battling other players for territory.
https://www.lioden.wiki/territory

It doesn't affect the other player in any way, so don't worry about "stealing" territory. It also helps to use the Search function in Crossroads to find kings near your king's level with lower stats.

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You can get more territory by paying for it or going on a user's page, scrolling to the bit where it's a PvP and select the "territory" option as other people have stated above <3

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