A real-world, real-history example:
The Sound Dues
The Sound, known by the natives as the Øresund
[ˈøːɐsɔnˀ], is one of three natural waterways connecting the Baltic Sea with the ocean; the other two are the Great Belt and Little Belt. Of the three, the Sound is the most convenient for traffic, so that it was a very busy waterway since times immemorial. In the Late Middle Ages and the Early Modern period, the strait was controlled by Denmark; nowadays, the western shore belongs to Denmark and the eastern shore to Sweden.
The Baltic straits. West to east, the Little Belt, the Great Belt, and the Sound. Map by Ulamm, available on Wikimedia under the CC BY-SA 3.0 Unported license.
Political control of Øresund has been an important issue in Danish and Swedish history. Denmark maintained military control with the coastal fortress of Kronborg at Elsinore on the west side and Kärnan at Helsingborg on the east, until the eastern shore was ceded to Sweden in 1658, based on the Treaty of Roskilde. Both fortresses are located where the strait is 4 kilometres wide.
In 1429, King Eric of Pomerania introduced the Sound Dues which remained in effect for more than four centuries, until 1857. Transitory dues on the use of waterways, roads, bridges and crossings were then an accepted way of taxing which could constitute a great part of a state's income. The Strait Dues remained the most important source of income for the Danish Crown for several centuries, thus making Danish kings relatively independent of Denmark's Privy Council and aristocracy. (Wikipedia, s.v. Øresund)
The Sound Dues (or Sound Toll; Danish: Øresundstolden) was a toll on the use of the Øresund which constituted up to two thirds of Denmark's state income in the 16th and 17th centuries.
All foreign ships passing through the strait, whether en route to or from Denmark or not, had to stop in Helsingør and pay a toll to the Danish Crown. If a ship refused to stop, cannons in both Helsingør and Helsingborg could open fire and sink it. In 1567, the toll was changed into a 1–2% tax on the cargo value, providing three times more revenue. To keep the captains from understating the value of the cargo on which the tax was computed, the elegant solution was chosen to reserve the right to purchase the cargo at the value stated. (Wikipedia, s.v. Sound Dues)