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This is my first question on this site! So, I have been trying to figure this out. I have a port city that is located at the mouth of a river. The problem is that the river forms part of the border between the port city's own nation and another nation. How exactly would a city in this position defend itself? Thanks in advance for any help!:)

Okay, I've been asked to edit the question. To clarify, I am not really looking for any novel techniques, but rather what is historically accurate. The city does have existing walls, and the terrain surrounding it is comprised of grasslands and there is a really big rainforest nearby to the southwest. The technology of my world is roughly Middle Ages (though I'm thinking about going gunpowder era or late iron age). The city is likely moderately wealthy, considering its location. I have a map here for reference (the big black dot in the northeast is the city being referred to. also, warning: the map is unfinished and is thus very ugly in some parts and lacks a lot of details such as many of the settlements) http://i.imgur.com/vEPJS3q.jpg If any more details are needed, I can provide more (though it is limited, the city isn't as developed as a concept as I would like)

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    $\begingroup$ What era/level of technology are we dealing with? Bows and arrows? Early gunpowder? Modern day? Do they have resources to sink into building up impressive defenses or are they more on a budget? How big is this river, and is it a major trade route? $\endgroup$ – Nex Terren May 26 '17 at 15:42
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    $\begingroup$ I'm voting to put this on hold as too broad for the moment. @NexTerren pointed out several things that you need to edit your question to address for this to be answerable. If the question gets put on hold before you have a chance to make those edits, don't worry; if you edit while the question is on hold, it will automatically enter the reopen review queue. Be mindful of that discussed in worldbuilding.meta.stackexchange.com/q/4868/29. You can take the tour to learn more about our format, and review the help center for more details on how we do things here. Welcome to the site! $\endgroup$ – a CVn May 26 '17 at 15:48
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    $\begingroup$ Okay! Thank you! I guess I'll get on that now! (might take a while though) $\endgroup$ – user38665 May 26 '17 at 15:51
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    $\begingroup$ If you could add a map showing the distances and positions that might help too. It might also help if you specify whether you want us to come up with novel techniques using whatever tech you have or whether you just want to know what is historically accurate. $\endgroup$ – Lio Elbammalf May 26 '17 at 16:01
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    $\begingroup$ @Palarran When the OP has stated they're going to make changes to the question it is usually best to hold off on answering until these changes have been made - your answer may be made redundant after the edit. If you required more information to answer properly it is best to wait for that information first. $\endgroup$ – Lio Elbammalf May 26 '17 at 16:53
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First, I'd suggest clarifying your existing defenses. You say it's a port city: does it already have walls? It should, given its long standing (being a city by default means it's been there for some time) and on a country's border at that, but you don't specify. Lacking any specific technology level also makes this difficult to answer specifically, but I'll assume medieval technology.

  • Exploit the river. That is arguably your single best defense that can and should be used to great effect. How is the enemy getting across that river to attack the city?
    • If by boats, then open fire on the boats and their passengers as they cross. Bows alone will do nicely for killing a lot of soldiers, but if you can get some catapults or trebuchets, all the better. The time needed to construct all those boats, apart from the preparations probably being highly visible, means you have all kinds of time to call for aid and generally prepare yourself.
    • If the attackers are going to be using some established bridge (which should in all rights exist, given that this is a port city and presumably a major trade hub) near the city, then the obvious solution is to collapse the bridge and deny it to the enemy; bonus points if you can reliably arrange for it to collapse right when the enemy is crossing over it (maybe just by turning some catapults on it). You force them to either construct boats and cross the river that way (see the previous point) or else go some way upriver to the next bridge; either result buys you time, and time favors the defender who can call for help and get a relief force to come their way in this scenario.
    • A port city at the mouth of a river is presumably dealing with a navigable river; it's probably too deep to be crossed on foot. If for whatever reason the river can be forded at some close point, that's a little unfortunate for your city. You could, however, throw up some fortifications at that point, spike the river with caltrops (metal or wood, they're still going to suck for the enemy, and the river will conceal them until the first wave starts screaming as they step on them), and generally make the enemy pay a steep price for the crossing. Five hundred infantry (archers and spearmen) at a ford littered with caltrops is basically impossible to rush; the enemy will have to clear all the caltrops before they can charge, and you're going to be firing away with your archers the entire time. You can still retreat with minimal casualties at the end, too.
    • EDIT: Seeing the map posted, the river's current is too strong to be fordable anywhere after all the tributaries have merged. Any bridges over the final stretch of river (likely only one or two) are going to be critical points to defend that should have been fortified long ago, being both defensive strongpoints and border crossings. Fording the two tributaries to come at the city from the west might be feasible at the cost of time.
  • Exploit your coastal access. Unless your enemy has a significant naval presence, they probably can't cut you off from the sea, and therefore cannot effectively siege you; if they can't mount a blockade, you basically win by default unless they storm your walls at a hideous cost in lives. You'd be almost impossible to starve out, and reinforcements could even be brought inside the city if necessary. Even if your enemy can enact a naval blockade to accompany a land-based siege, you're still going to be able to get some fish, and the river ensures a good supply of clean water for drinking and sanitation.
  • Your walls should be safe from undermining; that close to the sea, they're likely to hit the water table and flood their tunneling attempts before they can get deep enough. That river access also means a moat is quite feasible; if you have one or can quickly dig a trench for one, all the better.
  • The river also means your frontage for an assault is reduced; amphibious assaults with medieval technology are horribly dangerous affairs (see the boat-crossing scenario above) and very likely to fail even when done right, so their soldiers will be limited to a land-based approach. If the river is wide enough, the enemy won't be able to set up catapults on the other side to strike at your city effectively; even if they can, it's still not terribly urgent to mount a defense on that side, leaving you with more men to deploy to the relevant parts of the city.
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  • $\begingroup$ Good answer. Personally I was thinking of doing what the Byzantines did with Constantinople. Sure it isn't on a river, but the Bosporus acts similar to it. A rich and old port city should have high walls, with multiple layers due to city growth over time. A double harbor with sea walls and towers would help, but the greatest active defence would be naval. $\endgroup$ – user2259716 May 26 '17 at 18:23
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A proactive approach to defense would be really important for an exposed city like this. For instance, the nation may have a treaty established with the neighboring nation that limits military development and movement in the region near the exposed city. This could be backed up by economic strength (you rely on our food supplies during the rainy season, which we will cut off if you violate the treaty), military strength (we will attack your vulnerable area near the border if you violate the treaty), or cultural strength (the gods will punish you if you violate the sanctity of the holy land).

The nation with an exposed city could also use a combination of hard and soft power to establish the area near the river as a separate independent nation. They could provide aid to nationalist groups in the surrounding area in the hope of getting them to break off and form a buffer state.

Broader international diplomacy would be another powerful deterrent. The nation could establish friendly relationships with other nations and make it clear that it is in everyone's best interest that the city on the river remain in friendly hands. Belligerent moves near the city would trigger international backlash.

Good intelligence gathering and reconnaissance would be crucial. The nation would always want to have a good sense of their rival's military posture so that they could mobilize for a fight well in advance.

The correct defensive strategy in the case of war might be to abandon the city as tactically indefensible, but hold the line elsewhere and strike back at the rival where they are most vulnerable. Inflicting enough pain on the enemy, capturing their capital, ruling family, or important economic resources would be enough to force a resolution to the conflict on advantageous terms.

Investment in straightforward city defenses would be important, of course, but depending on the level of technological and cultural development, these other sorts of policies might be more practical than trying to build perfect city defenses.

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Everything that Palarran said.

Your major concern would then be troops crossing the river out of range of your city and then performing a land based assault. If you can put some small forts on the river going upriver to extend the range of your firepower to make them have to travel farther to get to the city.

If you are concerned with ships full of people attacking the city through sneak attacks, just put a wall between the docks & warehouses and the city itself. That way ships can offload into the warehouses without affecting trade but travel into the city will be restricted.

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