From the Wiki page on rent-seeking:
The classic example of rent-seeking, according to Robert Shiller, is that of a feudal lord who installs a chain across a river that flows through his land and then hires a collector to charge passing boats a fee (or rent of the section of the river for a few minutes) to lower the chain.
There is nothing productive about the chain or the collector. The lord has made no improvements to the river and is not adding value in any way, directly or indirectly, except for himself. All he is doing is finding a way to make money from something that used to be free.
I always assumed this was more of an analogy rather than describing what actually happened. (Note: he's not just "not adding value", he's destroying value.)
I recently read an article about the river Rhine drying up:
Kaub — known for Pfalzgrafenstein Castle, an imposing former toll station located on a rock in the middle of the Rhine — locals have noted the river’s ever-lower levels and are concerned about what the coming years will bring.
The name "Pfalzgrafenstein" is glorious, so I looked it up, and hey-ho...
The castle functioned as a toll-collecting station that was not to be ignored... Due to a dangerous cataract on the river's left, about a kilometer upstream, every vessel would have to use the fairway nearer to the right bank, thus floating downstream between the mighty fortress on the vessel's left and the town and castle on its right.
A chain across the river drawn between those two fortifications forced ships to submit, and uncooperative traders could be kept in the dungeon until a ransom was delivered.
Thus
a) making the feudal lord no better than a highwayman, and
b) reinforcing the point that all land value is ransom value.