According to Valkirah an Evony Rep, NPC's can not be owned in the strict sense of the word, for unlike a lake they can not be garrisoned. However, he did say that they can be "claimed." If you are strong enough to keep others away from it/them that is. For those who actually have armies so large that someone attacking and taking the resources from "your" NPC, maybe you should actually use that army to attack an unfriendly alliance instead of having that army sit on its ass. That's why Middle Ages rulers stood down the army after it was no longer needed. It was too damn expensive to garrison the damn thing.