Let's say you were the purple nation and wanted to most effectively control both the gap between the mountains and the sea, as well as the lands to the south between the mountains and the river.

Where would you place a fortification to achieve that goal? Or should those southern lands be ceded to the other nation because they are not worth the trouble of holding and the nation's resources are better spent solely on securing the gap?

Possibly Relevant Context:

  • The gap between the mountains and the mouth of the river is about 50-55 miles (80-89 km) according to the scale on my map. From the mouth of the river to the source is about 170 mi (274 km) "as the crow flies".
  • Through the gap and the forest (dark green) is the heartland of the Purple nation, with its capital and other major cities.
  • These borderlands are arable, but the Purple nation has plenty of farmland in the north. The biggest advantage of holding this area is to serve as a buffer against its eastern neighbor.
  • Both nations are remnants of formerly massive empires that experienced a collapse, followed by a renaissance. But neither is remotely as large or powerful as it once was.
  • They have a history of conflict. Green's founding and expansion was a direct cause of Purple's collapse and at the height of its power Green more or less fully conquered Purple and de facto controlled its entire territory. Green's collapse was driven by an uprising that ended with the founding of another nation (not shown) and forced it to withdraw inwards. Purple exploited those events to reassert its national identity and regain some of its lost land.
  • The two nations are currently in an uneasy peace and trade with each other. At one point relations between the two were friendly enough that Green was able to goad Purple into a proxy war against another nation, but that ended in a disastrous defeat for Purple and the monarch responsible is remembered as a disgrace, so the two are unlikely to ever enter into any sort of alliance or close partnership going forward.
  • I would rate the naval strength of both nations as middling. Green's military is overall likely stronger than Purple's, but Purple has the advantage of magic, which Green lacks.
  • Green is a feudal society and its lords have significant autonomy. Purple operates more like a unitary nation-state, with a much stronger monarch and central government.
  • Current technology level doesn't map neatly onto real-world history, but is a fantastical blend of Renaissance, Medieval, and Bronze Age characteristics. Purple leans more toward the Bronze Age end, while Green Leans more towards Medieval.
  • $\begingroup$ 1. Can you label the gap? Is it that glowy bit on the left edge of the map? Or is it where river meets mountains. 2. If you know where they are can you add cities? I am sure there will be one at the river mouth which makes that part easier. $\endgroup$
    – Willk
    Jun 8, 2022 at 1:05
  • $\begingroup$ @Willk I've added an updated image with the area circled. The map I cropped this from is a world map, so it only shows major cities. You can see one on the right (the green dot). There probably are numerous smaller cities and villages, but I haven't planned where they are yet. As for a city by the river mouth, that's exactly what I'm considering. Deciding whether to put it there or closer to the mountains, or whether to move the border so it cuts right across the gap instead of following the river (ceding that southern territory to Green) since idk if Purple can defend it effectively. $\endgroup$ Jun 8, 2022 at 1:17
  • $\begingroup$ Fortification as in? Barbed wires? Concrete wall along the border? Walled city in the location? $\endgroup$ Jun 8, 2022 at 7:49
  • $\begingroup$ @SparkShredder well, OP stated that the technology is no more advanced than Renaissance co I would guess that Barbed wire is out of question... $\endgroup$
    – Bartors
    Jun 8, 2022 at 8:13
  • $\begingroup$ Hello HumbleWanderer, welcome to Worldbuilding. For future reference, please note that this Q should be closed. "Best" is 100% subjective and any possible chance to describe "best" requires you to explain the conditions of your story (too story-based is a reason to vote-to-close). E.G., is the incoming army all cavalry? "best" is the top of the mountain. Does the city have a lot of chariots? "best" is the middle of the plain. Worse still, there are a ton of economic reasons that drive city placement (usually more important than military). Please keep this in mind. Thanks. $\endgroup$
    – JBH
    Jun 8, 2022 at 16:17

5 Answers 5


I belive that @o.m. is into something with his response but I will build a bit on it.

The location of the city and castle

I would place a heavy fortified city directly at the mouth of the river, this way the city have more or less direct control over the transportation on the river. The river will be an important trade route, especially since Greens have a big city further up-stream. This area is also relatively close to the forest, which means that lumber will be quite widely available. This will make the city a perfect site for wharfs and building ships, both for merchants and military. Additionally we should not forget that while the mountains block the access from the west, they may also provide, at least, stone for the buildings and preferably metals. Those should be relatively easy transportable to the city and out to the world (raw or as final product, the last option opens up for "industrialisation" of the area). The fact that the city lies downstream from arable land means that there should be plenty grain and other food available for the non food producing inhabitants of the city.

In case of war the city should stop the opposing forces from marching directly into the Purple heartland and bind up a sizable force as both the land forces and navy would be necessary to completely siege the city. If, on the other hand, the Greens should choose to ignore the city and march straight ahead into the heartlands then the city is an ideal outpost for raiding the Greens territory and wrecking their logistics and economy.

The border

The situation at the border is a bit unclear to me as I cannot see if the river follows the border and the river flowing from the east a tributary, or if the whole river turns east and the border on the southern part of the map goes through open terrain.

If the river follows the border all the way to the mountains then the border should be easily defendable as rivers typically posse a major hindrance in communication and troop movement. Some watchtowers and maybe a river flotilla should do the trick until the mobilization is complete.

However is the second is true than I can see two options. Either the Purple will build a wall (think Hadrian's wall) or other fortifications (if they can afford it) along the border. This secures the border but it effectively means that they have given up all hopes of further expansion in that region.

Or they could have a system of watchtowers there. It opens up the border for smugglers, small scale raids and even full scale invasion, but it also makes it easier to expand to the east and capture the Green city and secure the river as a natural border (which, imho, should be the geopolitical goal here).

Either way a secondary permanent fortification close to the Green city should be considered as it would create some kind of power projection onto that area and , as it would lie at the center of the border, could provide highly mobile units for fast response should there be a need for them.

The Borderlands

The borderlands itself remains me a bit of Ukraina (which by the way literary means "the borderlands") so maybe a social progress towards Kozaks could be seen there?

  • Have the city pay for the fort.
    Purple may be able to have the monarch build a fortress somewhere, but it is easier if he or she just has to appoint a viceroy and to supplement the paychest, and if the recruits and supplies come from the region. That means the fortress requires a city and a city requires villages.
  • Secure trade routes, not just borders.
    Depending on what that ocean/sea/lake leads to, it could be important to have a port city. Water transport will be considerably cheaper than road transport, and so it makes sense to "secure" the river, too. So it is a city on the river mouth. Read about the medieval staple right.
    If they can defend both, a bridge and bridgehead somewhere. Or would that be an eternal flashpoint, and they're better of with 'natural' borders on the river bank? Could there be a bridge, ferry, or fording site where both sides hold their respective river bank to tax traffic?
  • Depending on the size of the river, a flotilla to patrol it?
    These small river galleys would not prevent all smuggling, But they would notice an army going across or someone building a bridge. Think of the Roman Rhine and Danube flotillas.

A port city will form naturally at the mouth of the river, assuming the river is navigable and the mouth not too treacherous. With the technology described, rivers are the highways of the age. It's up to the policies of Purple and Green to determine which side of the river the city forms upon. (possibly both).

Another port town/city may form near the green dot where branches of the river converge. Again, it's up to the policies of Purple and Green to determine which side of the river the city forms upon.

The cities and towns along the river should be fortified. Not against a massive Green invasion -- that's unlikely. The feudal Green barons are unlikely to muster a large force for long enough (if they had enough wealth to do so, they wouldn't be feudal anymore). Therefore, the land gap itself seems not strategically important...the port city is the key location.

  • $\begingroup$ I'm probably projecting my own foreknowledge into the question a little bit here. The antagonist of my story is going to force the feudal lords to unify under his control and execute an invasion of the purple lands. So that gap will become strategically important, but on a second look you're probably right that it isn't seen that way currently. $\endgroup$ Jun 8, 2022 at 16:13

As described the lands to the south of the ocean are of subsidiary importance to Purple. Yes the land is arable and there are (or will be) towns and villages located there but according to your introduction the bulk of the Kingdoms population and it's wealth lies further north off map.

This makes the all of the land on your map to the south of the coast of subsidiary importance to Purple. In terms of Grand Strategy the North (off map) parts of Purple is what matters, not the South and from Purple's perspective all the land on your map is a 'nice to have' not a 'must have'.

That said and assuming the river is fordable at many points along its length the Greens can bypass Purple positions anywhere south of the coastline by either crossing the river north of wherever it is you decide to place a garrison or simply landing troops on the coast via ship. Either way any fortress and army placed by Purple in the South risks instantly being cut off from the main part of the kingdom and the enemy would control the way North.

This creates two strategic necessities for Purple. Firstly the need for 'Purple' to hold the coastline. Secondly the need to hold the open ground between the edge of the forest and the sea leading north to the rest of Purple's land and its major population centers etc.


As suggested by others a Fortified Port City that supports part of Purples fleet would be needed to oppose a seaborne troop landings or attempts to resupply an army from sea and yes the river mouth might well be suitable provided it is navigable to large vessels (many river mouths aren't, but that's up to you to decide). Anywhere on the coast with a good harbor, fresh water and fortifiable approaches would do.

The second problem is guarding the narrow open plain between the forest and coast i.e. the land route northwards. If the bay in the top left hand corner (11.30 on the clock face) of the area you circled has a suitable port site a fortress and port complex located there could serve both functions at once. If not you would probably need to build two fortresses (or a least a walled port and a fortress) that between them secure the coastline and block the way north via that narrow strip of land.

Point is if Green invades anywhere on your map as long as it doesn't seize your fortress/s all it gets is a lot of farmland which Purple can take back at its leisure. That narrow strip of open land leading north is the only thing that matters. Control that line of approach to & from the North and you defend the Kingdom! So if possible I would go with a single fortress/port complex on the bay at the 11.30 position.


Where are the trade networks? The fort(s) would be placed on/near the trade networks. See historical placements of castles and other pre-rail cities.

The mouth of the river would be the obvious place to start. Cross roads are always good places to consider putting cities. But depends on geography if it is a good location.


  • Is there high ground near the proposed location?
  • Is the ground solid? ie not mud flats
  • Is there resources to build fortifications/building near the site? ie less then 5Km.
  • How navigable is the river? Ie how large of a ship can travel the river? How far upstream? Bigger cross road is more important to defend.
  • Is there good anchorage? No good place to anchor will increase costs to make an anchorage.
  • Is there good spot for a bridge? (additional trade networks, but beware of cutting of ship access)
  • How good is the farm land?, The better it is the easier it is to have local economy support the defenses.
  • How much other resources near by?, the more resources, the more trade.

Other locations:

  • Further up river. i.e. Rome on Tiber
  • Nearby estuary. i.e. Venice near Po
  • Nearby port. Various fishing communities.

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