LadyEsme,
Forests, hills, lakes etc. are fair game provided they dont belong to a particular player. They are called valleys and if you click on it, it will show you the lords name if it is owned.
NPC's don't actually belong to a player. However npcs (barbarian cities) are actually created by players for the purposes of farming. The unwritten rule is that the person who built it should be the person who gets to farm it.
This applies to level5's and 10's. I don't think anyone cares about any of the other levels.
Its also accepted practice to farm an enemies npc's, effectively robbing them of food.