The narrowest point of the Tarkhon Passage—perhaps half a mile of water between Tarkhetan's walls and Torsnerhun fortress on the Astrelle shore.
Nothing passes the Needle without Tarkhetan's permission. Firemages on both shores can burn ships attempting to run the strait. The navy patrols constantly. Even if a ship somehow slipped through, the fortresses further east would catch it.
This chokepoint is everything. Tarkhon's wealth, power, and existence depend on maintaining absolute control of the Needle.