The Neck
The Neck is the sharp narrowing of the continent at Forgegate. Every known overland route passes through it, and that single fact explains a great deal of the world.
It explains why Forgegate is the richest trade city in the Known World: everything that crosses the northern landmass crosses here, and Forgegate takes a share. It explains why the war that produced the Pact of Reason burned Forgegate's farms though the city had no part in the quarrel — it lay between the two combatants. And it explains why closing the roads at the Neck starved Clearhold within a single season: the mountain valley cannot feed itself, and what feeds it comes through here.