In a feudal kingdom, a knight was granted a small island in the middle of a large river by the king. The island is only about 700 square meters. The island is too small to have any significant population or a meaningful economy by itself but the king promised that the new lord will be able to collect plenty of taxes. Unlike the Twins in Westeros, the estate includes only the island, although nothing is mentioned about the water sovereignty.

  • The geography, climate, technology and population density are similar to those in England, Benelux and Germany in the 13th century.
  • The island is located in the middle of an important river. It is the major crossing in the region. A bridge was built on it, linking the two banks.
  • The capital of the kingdom, a city of almost 25 000 people is located on the west bank, not too far from the bridge.

The island is still unsettled but our new lord has great plans for it. He wants to build a large castle made of stone (as large as there is space available). However, I wonder if this is possible. His only source of revenue will come form the tax imposed on people and goods crossing the bridge.

Considering that this is a "busy" road, can he realize his dream of building a castle? Could he have other revenues?

Bonus if he can do it without upsetting the merchants and the other nobles around.

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    $\begingroup$ Upvoted for being very useful to me to build realistically in Minecraft $\endgroup$
    – Mystra007
    Commented Sep 9, 2015 at 22:20
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    $\begingroup$ I thought that the knight would make money from taxing thise ships who pass. $\endgroup$
    – BlueWizard
    Commented Sep 10, 2015 at 5:50
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    $\begingroup$ 700 square meters would be roughly a circle with 30m diameter, but the island won't be perfectly circular. The castle walls alone could take more than a quarter of your real estate. $\endgroup$
    – o.m.
    Commented Sep 10, 2015 at 7:46
  • $\begingroup$ I agree with o.m., 700 m² is not much. You also have to build your road and two bridges on it. Not much more place for anything else than a dungeon. $\endgroup$ Commented Sep 10, 2015 at 12:52
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    $\begingroup$ Extremely important point: Rivers flood. Your knight better hope there's some significantly elevated land made mostly of rock on that little island or there's no point to ever building a castle on it. Unless he wants to be wiped out in the middle of the night without any warning sometime over the next few years. $\endgroup$ Commented Sep 10, 2015 at 16:34

9 Answers 9


He can absolutely achieve this goal, and do so without causing to much of a fuss (if he has the patience to not build it all right away).

Medieval castles were not built overnight, so there is little need to tax the proverbial pants off of the people. Castles went through stages, perhaps a wooden palisade to start and a smaller stone keep. Over time pieces can be upgraded with stone, towers and gatehouses can be added etc. This could potentially take the lifetime of the knight, but it doesn't have to.

Now, when it comes to paying for it the lord should not focus solely on taxing traders.

The purpose of a castle in the middle ages was to provide fortification sure, but more importantly it gave the lord a base of operations to control the surrounding countryside.

In such a location the lord is perfectly suited to develop and build a larger community around the castle, this would likely happen naturally in this instance so long as the lord kept the surrounding area safe from bandits and the like. Towns like a fresh water source and it is also important for farming.

A castle likely can't support itself for long without a supporting population center anyway.

Eventually it would evolve from a simple castle on an island:

enter image description here

To a castle with a surrounding town, which would result in surrounding farmlands and commerce increasing:

enter image description here

Eventually the lord could wall the surrounding city as well. This gives you another tax base (the local population) from which to pull funds for castle building. (oooh I added docks to the picture!)

enter image description here

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    $\begingroup$ It is possible to have a city around it but it would be on another lord's land. My knight only own the island. $\endgroup$
    – Vincent
    Commented Sep 9, 2015 at 19:20
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    $\begingroup$ This. And like 600 years later people will wonder why this metropolis was actually built $\endgroup$ Commented Sep 10, 2015 at 9:15
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    $\begingroup$ In the middle ages owning land did not give you the right to build a castle - you had to get special permission for that off the King. $\endgroup$
    – gbjbaanb
    Commented Sep 10, 2015 at 12:16
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    $\begingroup$ @James, you are describing the history of Paris! :-) $\endgroup$ Commented Sep 10, 2015 at 12:50
  • $\begingroup$ oh James, your art skill rival that of Picasso $\endgroup$
    – TrEs-2b
    Commented Jun 8, 2016 at 19:01

Diversify the knight's income streams and it shouldn't take long

The knight has a really great location and the simple answer is to charge tolls to cross the bridge. That works but it doesn't tap into the many different revenue streams that could be realized from such a great location. Besides, everyone hates taxes but if you're providing a valuable service then the mark up can be far far higher than what you'd get from taxes alone.

Build a castle that's a "Fireproof" warehouse then charge storage fees. Castles have always been a place to keep valuable stuff. Offer that kind of security to merchants so they can store their products without worrying about it.

Invent Fire Insurance and insure your warehouse/castle though you'll need to invent accounting too. Give merchants the option to pay you a reasonable fee so if their goods are lost while in your castle, you will pay them the cost of their goods. If you build your castle right, you shouldn't ever have to pay out or pay out very rarely.

Offer docks on your island for merchants. If a merchant knows they can offload their cargo directly into secured storage that decreases their risk. It's also good marketing and increases the emotional association between the knight's castle and security. You can charge large fees for docking up. If so inclined, have a trusted set of longshoremen to load and unload the cargo. That's even more money to be made.

Secure River Rights If the knight can secure the water rights to the opposing shores, then he can build a large series of docks that will increase commerce through the area. The capital outlay to build the docks may be high but this investment can be recouped in a few years if commerce is good and the king isn't anti-business.


Yes. It has been done before.

Religious Sentiment

Your nobleman could include on his teeny-tiny plot of island a religious structure and within a generation, it could become of significance to travelers. For a small fee, they can receive their blessings; for a larger fee, they can be assured safe travel. This is not dissimilar than the autonomous, but powerful Mont Saint-Michel, which at times served this purpose.


Not just people, but animals as well. Wealthy travelers would welcome a safe and fortified location for a stop along their way, whether for trade, pilgrimage, or noble duties.


A secure castle like the one you're describing, with such defense, is a formidable place to hold your kingdom's treasury. Although autonomous under your prince, there will be wealthy and powerful benefactors who would risk anything to protect you. This was done at a place I visited in Lithuania called Trakai Island Castle.

Natural Resources

While you're describing a river, there are places with precedence in the sea and on freshwater, with close access to natural resources. Bahrain and Delma Island (in the Arabian Gulf) have been written about for centuries in 15th century Venetian literature for their access to rare pearls. Select a natural resource, maybe even gold, that is precious and found in your river that you have autonomy over. Exploit it.

Be careful

Be careful with your size restriction. There are restaurants with the size of your castle's footprint. If your story allows it, consider expanding the floor area. If not, no worries: I've been to a tiny, island castle: they do exist!


To join the chorus: It is a great location. And business is about location, location, and yellow rubber ducks. Which did not exist in the Middle Ages and can be ignored.

First, if a king gives you an island with two bridges that connect the two banks of the river, it is because he wants you to protect and maintain the bridges and protect the boat traffic on the river. So regardless of who controls the river you will be expected to collect tolls on both the traffic crossing the bridges and going under them to pay for the maintenance and soldiers needed. You would also be expected, if not outright required, to fortify the island to secure such an important strategic location, and would probably receive some royal stipend or valuable privilege to pay for that.

So you absolutely could get your castle. Althought it would probably would start as wooden palisades and gatehouses. Rely on good location instead of expensive masonry for defense. This would then piece by piece be upgraded over time to stone construction. Although for an island, barring some specific threat, such as a bordering state with large army, an actual castle might not be necessary. Much depends on whether the river is navigable by warships.

Also note that the location really is valuable. Unless it is right next to a city it would be a secure stopping point for merchants. Brigands were a real problem in medieval times and an island with a palisade and armsmen for protection would be very welcome. Especially if you have to stop to pay tolls anyway.And people would pay for protection and food and drinks.

Also for traders going away from city, basic supplies might be cheaper to buy outside the city, so the island might be a good spot to sell supplies.

You can also have a transshipment business. Transporting cargo and selling and buying it in the city are two different skill sets. If business is good enough for merchants to specialize in one or another a secure location near the city for the cargo to change hands would be needed. The lordof the island might even take care of the transport between island and the city himself. The king might have been persuaded to give him some trade related privileges for transporting goods between the island and the city... Such privileges were a popular method for kings to pay for strategic castles or maintenance and protection of trade routes since it isn't necessary for the king to part with actual money.


It's a good spot!

The knight can impose taxes both on traffic crossing the bridge and on boats passing under the bridge.

Such a construction was perfectly legit in the middle ages. It only became offensive when a royal permit was missing and/or for exorbitant rates.



First any tax will upset nobles and merchants.

But not only will he be able to tax those using the bridge, but those using the river to transport goods up and down river as well. Having a castle/defensive structure on the island would give a bit of power for backing up the tax on both fronts. The biggest issue is can a large defensive structure be build on the island? many islands are sand bars, which would be useless for a castle, but if it is there because of a rock outcrop that hasn't been washed away by the river then you have a reasonable chance of being successful.

Of course having cannon to both threaten and protect will help a lot. One thing that would reduce the unhappy merchants and nobles would be if your taxes are fair AND you use your power to keep banditry down around the bridge and along the river on both banks in both directions.

Cannon overlooking the river and possible chains across the channels that can be raised and lowered to as threats to river traffic to stop and pay their dues.

  • $\begingroup$ Yes, the island is solid enough to support the structure. What is the best way to control the boats? Do I need to block the boats/river? google.com/imgres?imgurl=http://vignette3.wikia.nocookie.net/… $\endgroup$
    – Vincent
    Commented Sep 9, 2015 at 19:23
  • $\begingroup$ @Vincent threats with the cannon, but you can also put chains across that can be raised and lowered... I can add that $\endgroup$
    – bowlturner
    Commented Sep 9, 2015 at 19:35
  • $\begingroup$ No cannon yet. I suppose it could work, although it seems complicated to manage efficiently. $\endgroup$
    – Vincent
    Commented Sep 9, 2015 at 21:01
  • $\begingroup$ It's really more of a threat. unlikely you'd have to sink more than one or two a decade... $\endgroup$
    – bowlturner
    Commented Sep 9, 2015 at 21:06

I'd like to join the chorus but if everyone does, things get boring, so I choose not to:

Your knight got screwed.

He has a small island but no "anchor points" on either side of the river. The existing bridge likely already has an owner (There are other lords on either river bank, right?) who will be displeased to have the bridge taken away.

The king promised "the new lord will be able to collect plenty of taxes" probably without ensuring to keep it. After all, he said nothing about water rights. Well that's politics.

However his situation is not too bad, if he's clever (and a bit ruthless)

First he needs to find out the status of the bridge.

  • Is it good or will it need repairs soon?
  • Are there any claims on it and if yes, how strong is his claim in comparison?

Finding out the claims does not need to be easy. There's no central register. Any close city might have something in their records but a written contract in some private drawer might be lurking around, too.

In medieval times credits were usually paid back by nobles in terms of "royalties" (pun likely not coincidental but etymological). The noble got money now in exchange for lending his rights at something later like a source of tax income for a certain amount of time. This means the bridge as a tax source might be lend off to a rich family (the forerunner of banks) only to fall back to him after the time agreed in advance expires. The bank gambles on the income in the near future. They will calculate carefully to make sure they'll get their share.

This might already be the case. If so, the bank has a strong but time-limited claim. If not, any other claim (by other lords) can be circumvented:

The knight should address the king with concerns about the state and load of the bridge. He proposes to reduce the load on the already run-down (even if it's not) bridge to allow for repairs, if he gets in writing the right for taxing any new and old bridge located once the length from his island to the capital downstream and twice upstream (supposing the capital is located upstream, otherwise vice versa) regardless of previous arrangements.

If he's able to get that, he has royal decree that nullifies any claims and ensures he will always have the closest bridge to the capital. Plus he doesn't need to repair the old bridge, because he only assured to allow for not actually pay for repairs on his own.

Next he gets a credit lending out tax rights on the old bridge. The bank is bound to ask for the use of the money: "Constructions, I want to invest in my land to make it prosper by attracting more trade." The bank will likely agree, because more trade means more income from the bridge, which they will get.
Now the old bridge isn't of any concern for him. He already got a fixed share of the money beforehand. If there are any claims: Sort them out with the bank yourself!

He invests the money into building a new bridge. The bank will be pissed, because he'll be cutting their tax income with the new bridge: "But I said I'll invest in constructions!" With the two concurrent bridges, tax will be low attracting many merchants. The knight can increase the tax, as soon as the old bridge falls back to him.

In case there are strong claims over the old bridge, he can also sabotage it. He should only do so after the bank's claim expired. No need to anger them too much.

Now for water rights: He can have some bandits coming along the river. Attracting bandits shouldn't be too hard. If everything else fails, have some loyal men play them.
The bandits anger the king a little. And the knight reports his watch saw a boat escape along the river around that time. However he was unable to do anything and still is, unless he's allowed to deploy some chains across the river to stop and check travelers.

This should clarify water rights right away.

Now the Knight owns two sources of income:

  • water rights
  • new bridge (low tax at first)

He angered the bank but pleased the king (securing and extending an important transport route, defending against future assaults from the river) and merchants (low tax). Investing the income into building his castle step by step is the feasible way to go. It's unlikely he gets any credit after the stunt he pulled on the bank.

As an alternative, agree with the bank to finish the new bridge after their claim expired. This will keep them happy and taxes up. However it will attract less merchants.


A 700 square meters island is smallish (20m * 35m), but it is a good start.

If you wish to build a castle, and extend your influence, you will:

  • have to increase your revenue streams
  • extend the surface on which you can build

Let's start off with revenue streams.

First, bridge tolls. In the medieval era, in order to cross a (deep) river, there were only two solutions: bridges and barges. Barges were inexpensive to setup, but they had a very limited throughput, required a calm river and getting horses on them could be a pain. As a result, your bridges will be highly prized by merchants and travelers.

Second, river tolls, if the bridge arches are high enough for traffic to pass under then you can collect tolls in order to maintain the bridge. Note that medieval bridges generally had low arches, so only modest boats would pass under them. Make it so that the bridge piles have a slight "outcrop" just above water level where your toll collectors will sit all day long.

Note: if your castle is close to the sea, then the ability to shut down the traffic upriver can be valuable security-wise; Paris was invaded by Vikings because it lacked such protection...

Third, transfer fees. As mentioned, medieval bridges had low arches. As a result, higher boats will not be able to pass under it. That's a good opportunity for business! Rather than apply a toll, you can instead foster independent transfer crews... which will pay you a license fee of course.

However, with 700 square meters, that's about it... so we need to look how to extend this.

And here is the perfect answer:

Ponte Vecchio, Italy

When all you have are an island and two bridges, you need to make the most of them. In our case, this means extending them, gaining space on water:

  • you can extend the island (in the axis of the river)
  • you can extend the bridge (in width and height)

There are several ways to extend.

First, moored boats. It's frequent today in Paris or Amsterdam for example to have barges moored on the river side. You can even rent some... but back to our island, since you are providing protection, it seems natural that people will start mooring their boats here. Well, why not a tavern barge? An inn barge? And of course, you collect rent/taxes...

Second, floatsam/piles. The natural extension to moored boats is to built on piles or floatsams. Encourage it, and once again collect rent/taxes.

Third, extending the island/bridges. You can either build yourself, or you can lower taxes/rents for people who would build it (for some time). Note that the larger "stable" surface you have, the larger "unstable" you can get, and the more activity of course.

  • $\begingroup$ Nice picture, where is that house-bridge? $\endgroup$ Commented Sep 10, 2015 at 11:02
  • $\begingroup$ @GuntramBlohm: This is Ponte Vecchio, in Italia. $\endgroup$ Commented Sep 10, 2015 at 11:39

River, not the bridge

In such a medieval situation, the river would likely carry more trade than the bridge, and have less competition - i.e., road traffic might be diverted to another bridge, or even cause another bridge to be built, but major rivers often have no good alternative. Thus, no matter what the neighbouring lords do, they will be able to extract tolls from the traffic flowing along the river.


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