The Tarkhon Strait is the only sea passage between the Middle Sea and the open ocean. The inner sea has one door, and it is here: a channel running due west, where the broad waters of the Middle Sea pinch to a few miles of navigable water before opening toward the Western Isles. Every cargo moving between the Middle Sea kingdoms and the wider world comes through it. The alternatives are days of open-ocean sailing in dangerous water, and for most hulls there is no alternative at all.
Tarkhetan, the imperial capital, sits directly on the narrows. Its navy and its fire-mage garrison make the toll a fact rather than a request. A ship reports, declares its cargo, and pays. A ship that refuses is burned to the waterline, and the empire has done it often enough that it rarely needs to anymore. A hull pays going out and pays again coming home. That second toll, the one charged on a captain returning to his own sea, is the insult the Middle Sea coast has never forgiven and never found a way around.
The strait is the first of three tolls on the long corridor to the Shattered Sea, and the heaviest. Past the narrows a hull crosses the Sea of Merchants to the eastern edge of the Western Isles, where Sheîr sells the pilots and charges for the approach, and Tollgate taxes the passage through Phyndarr Sound beyond. Tarkhon takes the largest cut and takes it first, because it holds the one gate none of the others can open or close.
Pay at the narrows or burn at the narrows. The empire does not haggle, and it has never had to. — common saying among Middle Sea captains