Invisible boundaries. If you are gonna wall of areas, use chasms, walls or something that might actually make sense. And if you wanna go into more detail on why said chasm/wall is unpassable, make them inclined/whatever to make them unclimbable or whatever. OR put lava in the chasm. I don't care, just make a logical excuse why I can't go somewhere other than invisible walls outta no where UNLESS a wizard/spellcaster puts them there in the middle of a fight or something for THAT area ( and NOT THE WHOLE DAMN WORLD)