In the Wild, you can find other players' cities dotted around. Attacking a city will cause you to fight that player (exactly like an Arena duel). If you win the fight, you will take 10% of that player's gold, and gain a quantity of Kyanite depending on that player's level (you gain 50 kyanite per level of the player you defeated).
You can loot cities a maximum of 5 times per day. Additional attempts can be added for 395 Balens each.