Spine Mountains

The Spine Mountains are a continent-spanning mountain range which divide the western Known World and the eastern Unknown World. They are theoretically passable, but nobody really likes to do it, and therefore they don't. The only easy way to transverse the mountains is through the Orcish Pass at the north end, near the City-State of Thrownin. However, that pass, true to its name, is often perilously full of orcs. So, good luck with that, I guess.