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Let's say the city covers about 40 square miles (104 square kilometers), that machinery and some oil refineries are still usable (for construction vehicles), and that there is a lot to salvage in way of constructing walls. In addition, let's say one would have to be athletic to outrun infected humans. The intended city is the remains of Simi Valley, suburban sprawl rather than a concrete jungle like Downtown Los Angeles and surrounding regions.

Infected humans have a sense of numbers, time, distance, and self-preservation; they can learn from observation; and they can use tools, crude as they are and limited by their physiology, provided someone is insane enough to capture infected. Nonetheless, they aren't silent hunters and are limited in their cognition. Basically, they're comparable to the infected in the I Am Legend film; they can use traps but wouldn't devise one without first witnessing one.

Would it be sensible to build a wall around the entire perimeter, or would it be more sensible to set up outposts outside the perimeter, wall off a number of sections in the city, and reinforce buildings outside the sections to be used as pillboxes of a sort? The wall(s) is mostly to keep them out rather than humans.

Update:

Your responses made me reconsider some details in my world--for the better--and are therefore appreciated.

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    $\begingroup$ It would be nice if you could tell us a bit about the post-apocalypse. This could mean anything from biblical stuff to nuclear fallout. It sounds like "zombie apocalyspe" but I'm not aware of many stories set in such a world where an entire city is still intact. And please answer the following question: Why do you think it would be useful? One doesn't just build a city wall. I think I can extrapolate a bit what you are aiming at, but please clean up that question because I think it will be very popular if asked the right way. $\endgroup$ – Raditz_35 Jul 3 '17 at 13:19
  • $\begingroup$ What is the danger your people want to defend against? $\endgroup$ – Mołot Jul 3 '17 at 13:42
  • $\begingroup$ Medieval-style curtain walls are vulnerable to 18th century artillery. For protection against mid-19th century artillery you would need polygonal fortifications; many European cities surrounded themselves with fortified rings consisting of such underground forts and intermediate batteries; see for example the fortifications of Paris in the 19th and 20th century. Against modern means of attack such as ballistic missiles you woud need something else entirely. $\endgroup$ – AlexP Jul 3 '17 at 13:46
  • $\begingroup$ @Mołot I added some details about the infected, the primary reason for the walls. $\endgroup$ – user39368 Jul 3 '17 at 14:10
  • $\begingroup$ How does the infection spread and how fast can it be? If it is very infectious, a wall might just become your coffin. $\endgroup$ – KC Wong Jul 4 '17 at 2:59

15 Answers 15

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This depends entirely on your city.

  • If it's miles of suburban sprawl with wide gaps between houses and large gardens, forget it, go somewhere else.
  • If it's close and neat grid aligned tower blocks then you just have to block the spaces between the towers and you're done. That's a lot less work than the 20+ miles of wall you have to build otherwise.

What you really want to do is abandon the city, where all the people were and hence all the zombies now are, and go out into the deep countryside where the people weren't and hence there aren't any zombies.

If you want to build a wall around a city after a zombie apocalypse, it's to keep the zombies in not out.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – HDE 226868 Jul 5 '17 at 20:56
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A wall is as good as the men (and/or women) defending it. How many people is living there is going to be the deciding factor on wall's size. If there are too few, a large wall is a liability. The enemy can disguise its movements and make a sudden attack on a specific point of the wall, the defenders won't make it on time.

If there are thousands of people, you need a lot of space for them, and thus it may be a good idea having a wall which protects all your inhabitants instead of just an elite of them. Even if they all gather in a fortress, having to abandon their houses and farms to the enemy is going to take a toll on their moral - and on your supplies.

However you say something about "outrun infected humans". If we are talking about a sort of zombie apocalypse, we can more or less safely assume that the enemy is not intelligent nor capable of using machinery, and then, the principles still stand, but they can be more relaxed.

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  • $\begingroup$ Citation needed ;-) $\endgroup$ – Ghanima Jul 3 '17 at 21:29
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Realistically, in a post apocalypse, cities would probably be abandoned. There is no source of food in cities, once whatever was on hand runs out, and no way to make food. I don't have a link to exact figures, but I believe that without resupply (which would probably be the case in a post apocalypse scenario), the average large city with half of its population intact would run out of food in around three to four weeks, probably sooner for the majority as the few that had access to food stocks would hide/hoard them. Also, fresh water would begin to run out if the water plants were not operating.

This is one of the flaws in apocalypse style films that have survivors staying in cities. That is a Hollywood invention, to put the protagonists on ground that is familiar to the majority of viewers.

Most likely, the survivors would head out into the country, where subsistence food may be obtainable: game, crops. Also, survivors would be more likely to find a steady supply of fresh water in the countryside than in a city.

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  • $\begingroup$ I'm aware of the vulnerability cities possess. The intended city is what remains of Simi Valley, vastly depopulated during the apocalypse, and I handwaved the accessibility of groundwater to justify the presence of farming; I made a mistake not including that detail in the original post. Most cities are intended to be scavenger and raider dens. $\endgroup$ – user39368 Jul 4 '17 at 1:09
  • $\begingroup$ Yes, it might be possible to wall off a valley, but who will build that wall? Going to take a lot of people, moving a lot of material, and unless you really want to stretch fiction, they probably won't have heavy equipment to do this with... they'll run out of fuel. It might be more realistic, in a zombie type scenario, to have then going to a large island instead. Natural wall with the water. $\endgroup$ – tj1000 Jul 4 '17 at 18:41
  • $\begingroup$ I have since--posting this question--decided to stretch out the apocalypse so to provide long enough lasting (relative) stability that would allow the construction of walls. $\endgroup$ – user39368 Jul 5 '17 at 5:03
  • $\begingroup$ Up-voting this for referencing the lack of food. $\endgroup$ – CaM Jul 5 '17 at 16:48
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Building walls take time that you probably don't have a lot of when being beset by the infected. (Depending on the severity of you apocalypse, but I guess we're talking the standard zombie 70-90% of the population here.)

But sensible? Yes, definitely. As long as you subscribe to the theory that people get together under pressure and cooperate in a crisis. Walling in your living space and farmland makes it easier to keep safe and frees up resources that otherwise would have to be spent on defense.

You could feasibly start off with smaller separated enclosures that gradually grow and connect with each other over the years to produce something like what you want over the years. As long as the infected don't master siege weaponry or the art of ladder-making the inhabitants should be able to live inside their fortress with regular patrols along the perimeter (cleaning out anything that could be used to climb the wall such as huge piles of corpses and making sure there are no weakenings or breaches).

My money would be with one of the European cities who still have more or less intact medieval walls though. Such as Saint-Malo intra muros or Visby. (Make mental note: write a Zombie apocalypse story set in Visby before someone beats me to it...) Because building a pallisade or surrounding yourself with chain link fence is one thing. But when it comes to non-siege warfare nothing beats a ten meters high stone wall, and those take time to build.

Considering what you've told us about the infected I suppose a spike filled moat could be carved out with access to the proper digging tools and then you can errect barbed wire and chain link fences to get a first defensive perimeter. But it is risky work while being onset by hordes of ravenous zombies. You'd have to figure out a way of protecting the workers who will no doubt attract a lot of unwelcome attention.

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It is very viable in some circumstances, not at all in others. Your question is very unspecific and broad so one could easily write a book about it, but consider some things:

a) If you have city states that do trade with each other and have different legislation, over all a low level of cooperation: walling a city could be very useful to ensure you know who enters your city so you know who has to pay what fine for entering and in order to capture criminals and so on. A city wall in that case would be mainly used to ensure everyone knows where a city ends.

b) If hand guns are still available, a city wall becomes counterproductive. One of the best ways to defend a city with modern hand guns is to let the enemy into that city and hide in the houses instead of letting him know where you are (on the city wall, right there, I can see you and now you are dead!). I'm sure you've encountered the concept of urban warfare before.

c) If you have enemies, and it sounds like you do, that get stopped by a wall, of course a wall is a good idea, but one should look into other, more cheaper and maintainable solutions such as fences.

I think details are needed to tell you how well a city wall would do in your world specifically.

In any case, I would advice against walling an entire city. Just wall the bare minimum of space. So I guess the answer is always no - only parts of a modern city. As for details how to use houses in your wall and so on, just check out the Berlin wall. It's a mediocre example because the post-apocalypse (whatever that may be) has very different demands and resources available, but they build a wall in a modern city ...

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  • $\begingroup$ a) There are city-states that trade with each other. b) The wall is more for the infected though I did consider the advantage of using buildings, which is why I included the option of having buildings fortified. Keep swift moving, cunning infected out vs design with humans in mind, allowing the possibility of the infected entering? My issue. I will look at the Berlin Wall; thanks for the suggestion. $\endgroup$ – user39368 Jul 3 '17 at 14:18
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    $\begingroup$ @X22T7V This is a bit off-topic, but you should be aware that those "infected" wouldn't be a threat in reality, so don't overthink. I personally wouldn't worry too much about doing it as realistic and strategically perfect as possible given the premise. Two examples that are super popular but have HORRIBLE military strategy: LotR and Harry Potter. If you think a wall is cool, do it. If you want to know if it makes sense vs zombies: They are no real threat, so yes, but there are cheaper options but why not. It is also already established in the post apocalyspe, eg in the Fallout games. $\endgroup$ – Raditz_35 Jul 3 '17 at 21:09
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What good is a wall against an omnipresent threat?

Castles only worked because 99% of the time, you could freely leave your castle or let people in; which you would need to do to get food from farms in the surrounding countryside. Castles would often stockpile food in them so that some people could hole up in the castle for a year or two before they run out of food. But that's how sieges worked. You surround a castle and don't let any food get in until the people inside starve to death.

If zombies are going to be around for long enough that you've got time to build walls, then it's too long to just hole up in a small walled city. How are you going to protect your farmland?

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You will need a city, and you will need farmlands.

The city needs to be as compact as possible. Select a number of city blocks with enough living space for your survivors, block all the roads, block all windows and doors on the outside, add barbed wire, demolish any nearby buildings to have a clear line of sight, post 24/7 sentries on rooftops, and you are more or less secure. But that's just your city.

The city needs to be fed. So you need a much large area next to your city, which would be much more difficult to secure properly. You can build a perimeter fence, but you can't make it as big as city wall, and you can't watch 24/7 every yard of it. So assume your farmland is not very safe, and any people venturing there would have to take appropriate precautions.

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  • $\begingroup$ Farmlands and food would be crucial to any post-apocalyptic scenario. $\endgroup$ – ChuckCottrill Jul 4 '17 at 3:55
  • $\begingroup$ The more tightly you pack your survivors, the greater threat you face from internal incidents (i.e. survivors turning into zombies and attacking from within). So "enough" living space needs to factor in some partitioning into internal fortifications and ability to both enable and restrict internal mobility as needed. $\endgroup$ – HonoredMule Jul 4 '17 at 17:23
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    $\begingroup$ Farmland on the other hand doesn't need the same kind of management. Its only security needs are limited to preventing damage of crops (which presumably zombies have no incentive to occupy when vacant) and keeping them clear of zombies while they are being farmed. Farmland is likely to be the easiest to (adequately) secure, especially if there's significant distance between home and farm. The path between of course represents a significant third consideration and possibly the most difficult one due to threat of ambush, surprise, or being cut off from the relative safety of home. $\endgroup$ – HonoredMule Jul 4 '17 at 17:26
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You already have walls. Because even before zombies people didn't like other peoples wandering through some important sites like refineries and factories enter image description here

So we make walls around them. Or fences. If these sites are close to each other we build footbridge. If they are far apart we build tunnels. If you are in post soviet countries your factories usually have even bunkers.

It's easier and more energy efficient to find good spot and adapt it than to totally convert chosen spot to your needs.

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The real question is why? Fortifications went out of fashion in the 20th Century because weapons development rendered them obsolete - however they are becoming notably prevalent in low-intensity conflicts.

Walling will therefore really depend on the level of weaponry in your post-apocalyptic world - if equipment such as aircraft or heavy armour still exists then they won't really help, if you're just up against small-arms then there's much more utility in walling things in. It is also worth considering the alternatives to walling; mines in particular may be an effective way of keeping people out.

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    $\begingroup$ Fortification never went out of "fashion" in warfare. Every single major conflict has involved fortification of one type or another. Still today we use HESCO barriers for practically every major position for which we have time/resources to set them up, and sandbag fortified pre-existing walls for lesser positions. A particular type of fortification may have gone out of style, but fortification as a whole has never stopped being relevant. $\endgroup$ – BryanGrezeszak Jul 4 '17 at 10:45
  • $\begingroup$ Apologies - poor choice of wording on my part. I meant that large, permanent fortifications have gone out of fashion. The protection of fighting positions is still alive and well, but generally on a very small scale and with heavy emphasis on camouflage. HESCO and similar fortifications are primarily useful in low-intensity conflicts, as mentioned above. $\endgroup$ – Matt Bowyer Jul 4 '17 at 23:33
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A 40-square-mile city would have a perimeter that is 25.3 miles. If there were a relatively small number of human survivors in this city, and only a smaller subset of them were helping with the wall-building effort, and they had only post-apocalyptic resources at their disposal, I would have to think that it would take several years to build this wall. So a few questions arise due to the multi-year nature of the construction process... -If these carpenters could survive for years before the wall gets erected, and furthermore, survive despite the vulnerable position of erecting a wall while under attack, then couldn't they just survive without building the wall at all? -If these carpenters can avoid getting eaten for years while building the wall, then will the zombies still be around years later? Who will they have been eating during the passage of all of these years? Might they zombies have spoiled and died of starvation if they go years without eating?

I think a more sensible approach might be to just seal the alleyways between a few buildings in a 1-block radius and perhaps give it a guarded passageway that is advantageous for defense.

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    $\begingroup$ That's a remarkably specific value for the perimeter of an arbitrary urban zone. Are you suffering delusions of accuracy? $\endgroup$ – Separatrix Jul 5 '17 at 8:48
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The problem is to protect your survivor population both during the initial outbreak, and provide security for long term survival. You also must plan for sustainable supply of resources. Assume that you can both identify survivors, and that you have sufficient resources to protect survivors during the initial hot contagion period (food, medicine, ammunition, et al). And assume that your leaders have both the foresight and the political willpower to enforce the necessary changes to preserve the citizens.

Initial protection period could be bootstrapped using existing police and soldiers, firefighters and construction crews. Assuming you have access to sufficient resources, you would assemble emergency barriers, choosing strategic locations to erect temporary barriers, leveraging existing buildings to provide part of the natural fortifications. The erection of barriers would proceed simultaneous with demolition of buildings and obstacles outside the barrier to enforce a kill-zone 200-1000M wide around the safe zone (how fast do your infected move?).

Since the threat is low-tech (infected, low speed), you would likely need to erect a barrier between 12-20 feet high. Temporary fencing would suffice for some areas (short term), but would require constant vigilance from defenders. Chain link is too easy for infected to scale. You need entrapping obstacles in the kill-zone. As the initial situation stabilized, you would continue to reinforce and improve existing barriers and erect new barriers.

You could quickly erect walls around parts of the city using existing semi-trailers and train boxcars. These obstacles could be installed as quickly as the survivors could drive or haul them into position. This initial barricade may only be 8-10 feet high, but augmenting the barriers with a 3-4 feet high battlement would protect the survivors from contact with infected.

Once the initial onslaught of infected has been resisted or repelled, ongoing fortifications could be constructed by trenching and obstacles in front of the walls, and as time and resources permits, combining demolished rubble with concrete to pour more effective walls. Remember, the threat is from infected, who have been reduced to primitive weapons (no guns or munitions). As the walls continue to improve, the trailers and boxcars can be dismantled to provide metal sheathing for the walls (again, no explosives being employed by threat).

Your initial city may have irregular shape, due to exigencies during initial fortification construction. During this stabilization period you would correct the perimeter to the best shape, especially as the barriers grew more effective. Guard towers would be erected to provide enfilading fire from height (again, no snipers amongst the infected). And you would also have no threat of engineers or sappers from the threat side to tunnel under your walls.

All of the classic problems with walls - explosives, siege weapons, munitions, sappers, trenchers, engineers - pose minimal challenge. The infected are a threat that does not pose serious problems to the classic weaknesses (other than needing distance and firepower). Walls need not address the myriad weaknesses that normal external threats pose. Think about the siege of Stalingrad as an example here.

Assume that the food, medicine, and ammunition challenges have been managed to this point, then you need to ensure that you have sustainable food and water supply, sewage, and industry. Cities are often located near water and sewage, and we could assume that a nearby power generation (hydroelectric, nuclear, geothermal, or solar) could supply sufficient energy for the security lighting, and reconstruction efforts.

Food is the problem. You cannot grow enough food inside the city walls to feed the population. The land is likely not sufficiently arable, the livestock was not kept near the city. You have too many people for the land, and you need enough people to guard the walls 24x7, and provide all the industry you need to sustain the city. You could assume enough recycling efforts to provide materials, and your city might be located near a quarry or mines, providing ample natural materials close at hand.

What you really need are natural barriers. You need barriers provided by harsh or impossible terrain. Think the cliffs of Dover for example. Typical cities are located on rivers, at ports, with very little natural terrain providing defenses. Other than buildings there is little to impede the leisurely stroll to approach many modern cities. But San Francisco, Manhattan, or Long Island have limited access other than by water. Can you identify a city that provides natural barriers on two or three sides? Could you devise a way to make the water a natural defense to the infected?

Identify those cities which have substantial natural barriers, and your survivor cities would only need a fraction of the defenders that a city requiring a complete wall would require. Perhaps your city is located on a natural rise, and you can engage earthmoving equipment to cut a 20-50 foot high wall (faced with metal sheathing) around large parts of the perimeter. Perhaps the infected cannot swim, and water prevents their approach.

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    $\begingroup$ "Can you identify a city that provides natural barriers on two or three sides?" Simi Valley is surrounded by arid Southern California mountains and has few easily accessible roads. Walls are admittedly overkill under those circumstances, but I want to first scrutinize that city make sure I have a number of factors covered for when I consider less secure cities, like St. Louis and Memphis. And with StL and M in mind, would it be feasible to use some of the city (outer portion) for farming given the presence of the Mississippi River? It seems possible at a glance. $\endgroup$ – user39368 Jul 4 '17 at 7:01
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It depends mainly on the plan for the city. If it just a minor stay or you plan to strip the city of useful stuff before departing forever, outposts with some strong pill boxes will do fine. But if you plan to make this back into a functioning human settlement in a zombie world a wall is a must. With the looming axe of death at anytime by a zombie who may have slipped through the outposts, peoples work ethic and moral would bottom out. So even if they had factories or farms to work at, no one would go, especially after one or two braver people went and died that way, which would cause even more internal problems. A wall provides security to a populace within.

But building a wall around an entire city it too much all at once, it would have to be done in stages. Either expanding out from a central point, each new wall is built further out capturing more of the city and land for use at each completion would be a good way.

      +-------------------------------+
      |                     3         |
      |     +--------------------+    |
      |     |          2         |    |
      |     |   +------------+   |    |
      |     |   |    1       |   |    |
      |     |   +------------+   |    |
      |     |                    |    |
      |     +--------------------+    |
      |                               |
      +-------------------------------+
          Or 
      +-------------------------------+
      |     |          |         |    |
      |-----|----------|---------|----|
      |     |          |         |    |
      |     |          |         |    |
      |     |          |         |    |
      |-----|----------|---------|----|
      |     |          |         |    |
      +-------------------------------+

Something of the like, morely just extending the city boundaries a step at a the time to disperse the population into more useable space. But how much and how far are all dependant on the amount of people.

So to sum it up: If you are making this city into a place to call home, make it secure!

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I think you can look at real life examples of walling off large areas/cities:

These walls are all designed to keep out humans, but will work just as well on large wildlife and zombies. They are all at the kind of scale you are looking at. Their construction has been usually under adverse circumstances. By most metrics, they did their job. They are expensive in manpower (constructing and manning) but mostly low-tech (mines, some lights, sensors), and reasonably low maintenance.

Given these examples, I'd say walls are pretty well proven, and an efficient way at reducing the surface of potential attacks.

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Why would you?

If we take the population density up to that of New York City, then your city could support a million people. But where do you get food for a million people post-apocalypse?

Rather than walling off forty square miles of city, wall off less than a square mile of town and a much larger area of farms around it. So the central area is defensible at night while the larger area can be cleared and defended during the day.

You want the entire larger area to be within an hour or so of the central area. So people can leave the central area and reach the outskirts of the larger area in a reasonable commute. That gives you a larger area of around thirty square miles (assuming a walking radius of about three miles).

You wouldn't want to start with a city for this, as the city will have paved over the good growing area. You'd be much better off starting with a small town (or even something like a hotel with a few supporting buildings). Houses and city infrastructure are easy to find. What's hard to find is food.

For the kind of small communities that you'd actually have, you don't need a city. If you were in a city, you'd need to leave before you had time to do things like build walls.

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It really depends on your type of zombies. Slow shamblers can't get past a chainlink fence but the ones from "I am Legend" were perfectly capable of climbing and the zombies from "World War Z" swarmed over walls through shear numbers.

In "Wayward Pines" they used a wall topped with an electric fence to stop them climbing

Most likely you'd run a wall as well as reinforced buildings inside to provide a refuge and a pillbox should the wall fail.

If too aggressive for a wall, it would have to be underground linked pillbox defenses

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