The Karth Crossing

The singular fact of the Karth gorge that makes Karth Bridge matter: the gorge runs unfordable for thirty miles except at one timber span, and whoever holds that span holds the north.

A traveller cannot cross the Karth anywhere in the Northern Reach except at the ford beside the watch-tower. This is not a feature House Vandahl built — it is a feature the gorge already had, and Vandahl merely noticed it and put a tower over it. Lose the bridge to flood, fire, or abandonment and the shortest path to the foothills becomes a three-day detour. This is why one tired sergeant still holds the post, and why the tribes run their track along the gorge wall rather than crossing it: the north is divided between those who can cross and those who cannot.