A toll castle (WP) was a castle built either on a river, lake, or road intended to guard that trade route:
Such castles were usually placed at strategic locations, such as border crossings, river crossings or mountain passes, and were manned by armed guards. The actual toll-collecting point lay below at the road or river and was often linked by walls to the castle itself.
WP also has a list of these castles. Some of the descriptions are helpful in describing how they were situated/positioned in order to operate:
Albrecht commissioned him to rebuild the ruined castle to secure the passage of ships on the Danube. In 1438 Scheck von Wald received the right to tolls for ships travelling upriver. In return, he had to maintain the towpaths by which the barges were drawn upstream. In addition he built a toll house on the riverbank that now serves as a forestry house.
The castle of Chillon is built on the island of Chillon, an oval limestone rock advancing in Lake Geneva between Montreux and Villeneuve with a steep side on one side and on the other side the lake and its steep bottom. The placement of the castle is strategic: it closes the passage between the Vaud Riviera (access to the north towards Germany and France) and the Rhone valley which allows quick access to Italy. Moreover, the place offers an excellent point of view on the Savoyard coast facing. A garrison could thus control (both militarily and commercially) access to the road to Italy and apply a toll.
The castle functioned as a toll-collecting station that was not to be ignored. It worked in concert with Gutenfels Castle and the fortified town of Kaub on the right side of the river. 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. The dungeon was a wooden float in the well.
... a toll castle to ensure collection of tolls at the river crossing of the River Main and the borders with the state of Würzburg.
Later, in charter of Trenčín district administrator from 1358 is an explicit mention about castle and about a toll station collecting a toll for passage over the river Váh under the castle.
Further, recently, in a book ('Eesti keskaegsed linnused', K. Aluve) I saw descriptions of two (other) methods:
Ülejäänud neemikust Emajõe ja Luutsna jõe vahel eraldas linnust vallikraav, mis praegu on soostunud. Linnus oli rajatud piiskopkonna idapoolse veetee - Emajõe alamjooksu kontrolli all hoidmiseks. Vanadest sadamakindlustustest annavad tunnustust pinnasest välja ulatuvad puitposted. Jõeteed tõkestasid risti jõega kolmes reas 30...40-m vahedega paiknevad postid, mille külge sai kinnitada ketid või ujuvpalgid.
The height of the castle ruins from the river is approx. 6 metres. The rest of the hilltop was separated from the Emajõgi and the Luutsna River by a moat, which has become bogland. The castle had been founded to control the river to the east in the bishopric. ... Old port fortifications are proven by rows of wooden posts reaching out of the riverbed. The river was barricaded by three perpendicular series of 30 to 40 metres' separated posts to which chains or floating logs could be attached.
Ürikuliselt on Uus-Kastret mainitud 1392. a., mil konkreetselt on viidatud linnuse asukohal jõevee pinnale paigaldatud palgile, mis takistas laevu tollipunktist märkamatult läbi hiilimast.
Records have placed the Uus-Kastre [Warbeck] into 1392, when a specific mention was made to a boom placed on the surface of the river at the location of the castle, preventing ships from sneaking by the toll-booth.
These examples nearly always seem to rely on fairly solid methods of preventing river traffic (it could not be diverted). With land traffic, I imagine the primary problem would have been the difficulty of diverting this off-road (presumably heavily forested) -- but my primary interest is in river travel. The Aggstein example with over-land portage of ships also makes the toll-station relatively simple to operate (please correct me if I'm wrong).
Warbeck, Oldenturn, and Pfalzgrafenstein are great examples of river level castles. Most of the other German/Austrian examples, however, seem to be placed relatively high though the statement is made that the toll-booth was connected to the main castle by some fortified walls. This makes it sound as if such defenses-by-the-river were relatively weak -- in other words, more intended to cow than with actual effectiveness. WP entry on booms as a navigational barrier also lists numerous cases when ships were designed to destroy such booms (though all the cases are of battles and military vessels).
Pfalzgrafenstein puts the pressure on the boom/chain as acting as the main barrier -- with uncowed traders being taken into custody in the dungeons. Nevertheless, without an active method of endangering the ship where it stood, this sounds relatively weak as a method of enforcement, especially considering that some trading cities were very powerful (e.g. the Hansa union) and probably not interested in paying tolls -- unless these tolls provided a demonstrable good (which is perhaps easier demonstrated for Warbeck/Oldenturn where the castles would have also been the first line of defense for the inland trading towns).
I guess what I'm surprised by most is that there's no more active mention of:
- traders trying to run these blockades;
- toll-gatherers using heavier firepower (i.e., siege engines & cannons) to enforce the tolls.
While some of the descriptions did lead into robber baron theory (some good discussion of that at this question), most seem to have not. Therefore, was the general levied toll acceptable to both parties and arrived at communally? Or was it not worth it for the traders to upgrade their ships and the toll-castles to upgrade their measures to prevent the few on the other side who did either of those things?
While not necessary, if there is positive evidence of running past toll-castles that would be great supporting evidence in an answer.