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The premise is as follows:
A species uses biological constructs to protect themselves from forces that attempt to exterminate them. These biological constructs have a far faster growth than the species itself, so it settles worlds in advance of the species. Most worlds are terraformed as much as possible by a prior entity, worlds outside of the goldilocks zone are prepared with ease of living inside protected areas.

Species A is a humanoid group consisting out of religious humans who have their gods act through their priests while various supernaturals (werewolves, vampires, mages, various demons/angelics) live among them in relative harmony. So their culture is basically "earth with a twist". The danger of extinction has focused almost all efforts on survival.

The biological constructs, because of the religions of species A, are all humanoids build to combat perfection but without any non-biological implants to enhance them to keep them relatively cheap and use easily attainable materials (these are the guys from my "super soldier" questions). When not in combat their culture is a sort of hive mentality where they'll work in unison on any project to further the safety and population of species A. The cities and towns they build all have a religious hub that is build close to when the humans arrive, and are otherwise completely up to the bio-constructs to build and devise the layout.

Species B is a group of multiple species under thrall of multiple "big" gods who found mankinds persistance to cling to their own gods worthy of extermination.

As the bio-constructs settle the world, they prepare infrastructure for when their owner species arrive. Simultaneously they form a buffer between the hostile forces and the humans. So the bio-constructs build things to last against invasion. I first envisioned cities with city blocks that sport big artillery or cross-fire on each intersection, but this would be far too vulnerable to enemy large-scale attacks. So the question is how would these bio-constructs build up the world to be as hard as possible to exterminate in case of attack, and still make the world as livable as possible for the species once they have the population to start settling it?

Assumptions:

  1. The invading species wants the world to remain habitable after the invasion. No carpet-nuking or throwing asteroids so large the planet needs terraforming again (or more than a year of having to settle down before colonists can live there again).
  2. Orbital bombardment, similar to airstrikes, are long and expensive campaigns. Also it seems hard to tow enough mass to the planet to destroy its entire surface. Space-based airforces (that go into the atmosphere) would be more expensive to maintain due to their requirement to speed up and rejoin their orbiting carrier.
  3. There is FTL travel available, but due to it's workings you require increasingly smaller jumps the closer you get to a large mass (like a planet) to not exhaust your energy.
  4. The bio-constructs can construct food from the raw components, no need to grow it.
  5. The bio-constructs do have a space-based force, but most of it is used as reinforcements to planets or assaults elsewhere rather than defense. enemies would simply gather more forces than the standing spaceforces anyway and blow them apart, better use them as reinforcements to stop an attack. Due to the scale of the conflict, reinforcements would on average take 2 to 10 years to gather and arrive at the planet, giving the invaders time to mount a ground assault.
  6. Bio-constructs have access to ground-based anti-orbital equipment. This has it's flaws of course as these require energy to get it into orbit first. On the flip side, the ground forces have far higher energy available than the space ships. FTL-based defenses are also available, but these options can't "fire" its projectiles to a distance closer than 5km of the surface because the energy requirements go through the roof.
  7. Technology has advanced such that the combination of armor, weapons and countermeasures puts the capability somewhere around the 90's in terms of detection and firepower.
  8. Early warning time is 6 hours to a week, depending on what direction the attackers are coming from and the position of the planets (FTL needs to avoid traveling close to planets because lf the added energy requirements).
  9. The bio-constructs are all military capable.
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    $\begingroup$ Just to be clear: Species A (could use a name) is under threat of extermination from Species B (could use a name). Species A is trying to colonize to preserve itself. Species A uses a bio-something that pre-builds cities, defenses, and provides food once the planet is settled. The question is how the bio-something would layout a city for Species A under the given circumstances? If I'm right, please explain the physiology of species A and B. If I'm wrong, please edit to clarify your question (actually, do that either way, please). $\endgroup$ – JBH Jun 9 '18 at 19:38
  • $\begingroup$ @JBH not in a position for long answers atm. Species A is under threat. Species B tries to invade&exterminate. Species A uses bio-constructs to do this. For the purpose of the question (something that needs to be added) a 90's combat tech situation prevails (due to the total of science and countermeasures). Also, species A, species B and the bio-constructs are humanoid for ease of answering. $\endgroup$ – Demigan Jun 9 '18 at 19:46
  • $\begingroup$ Layouts of towns and cities depend heavily upon culture (about which we know nothing) and economy. Moscow and New York have very different layouts because those cities were built for very different purposes...and sometimes those purposes changed over time. We need to know much more about Species A's needs (Do they have huge ant-like fungal farms? Do they build underground? Are they afraid of heights? Do they dislike walking long distances?) $\endgroup$ – user535733 Jun 9 '18 at 19:58
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    $\begingroup$ How much early-warning do these bio-constructs get before an enemy arrives, and how fast can they grow responses to those enemies? If they get a 1 month warning and need 20 days to build up defensive structures, they'll take a different path than they would if they need 100 years to grow structures at hardwood-tree speeds, and the enemy can appear overnight. $\endgroup$ – Cort Ammon Jun 9 '18 at 22:17
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    $\begingroup$ In regards to assumption 2: a couple of "rods from god" are relatively easy to manufacture and deploy, as getting into orbit doesn't seem to be a problem for neither A nor B. Such kinetic weapons are near impossible to intercept, and can deliver a lot of localized energy. There is no need for the invaders to send ground troops; just drop (a lot of small) stuff until your enemy gets killed on impact our buried under fortifications. Note that this cheap method of warfare keeps the surface habitable. In regards to 6: if that's not a laser, it'll be easy to intercept or evade. If it is, 1/ $\endgroup$ – Orphevs Jun 12 '18 at 9:42
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First, I'd take a leaf out of the book of Alien: Covenant. If you've got a world that is populated already by a hostile, but non-advanced species, then a short-lived, biological weapon is the way to go. Unleashing a toxin that kills multi-celled non-plant life and creates large predators is perfect. Those that don't die in the original attack are killed by the mutants, who die off themselves soon enough when there is no food. It could be done as part of the terraforming process, and if your species can do that, then a plague of this nature is well within their capabilities and will leave the resources of the planet unscathed, apart from the fauna. You can bring useful ones with you and I'd assume that research missions to the planet have already gathered samples of any important organisms.

Right, city formation. As others have said, most cities are rather organic, building up as people live there and changing over time. Most aren't planned to a great degree, or aren't planned solely for defence purposes. But if yours is, it depends a whole lot on the technological level of the attacking species. Without that, it's difficult to determine so I'll say some general stuff. First, utilise the ground. With the technological ability to cross space, digging will be easy. If it's important, it goes underground. No exceptions. Have security doors and checkpoints regularly in the tunnels and they'll be exceptionally easy to defend.

Space everything out in a concentric circle pattern. Regardless of enemy capabilities, their main strength will be numbers. Essential buildings and technology, such as power generation, is in the centre, with less important buildings expanding out. That way, if an outer circle is ever breached or lost, forces can fall back to a smaller circle and regroup. The smaller area will be easier to defend and they won't lose any essential technology, because that will be at the centre. Can't go wrong with a wall either. Separate the concentric rings with walls, complete with doors that must be activated by the defenders. Sure, they might be broken through, but it will take time and give the defenders an advantage.

Another smart choice is to divide the circles into sectors, again splitting them up with walls. This again serves to limit and delay the enemy whenever they take a sector; when one sector is compromised, the rest of that circle is not. It makes it easier to limit the attackers and run counter-attacks. It can also be built without exposing the rest of the city: with one sector built at a time, the rest of the construction is secure.

Again, I cannot stress the underground enough. If interstellar invaders can colonise a planet, then they have the technological advantage. Build down or up. If you can cross space with such ease, you can build a terrestrial superstructure. If they have no weapons like missiles or nukes, build something a few thousand storeys high made of super strong carbon or something: you'd only have to defend the base against attacks. If they do have highly destructive weapons, go down a kilometre or so. You have the capability: use it.

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  • $\begingroup$ I've updated the post. I am also asking for ways to PREVENT a successful invasion. I would protect against viral attacks through volumetric counters: Any populated area creates a bacterial/viral biome that lives off other viruses and bacteria, any viral attack would have a small radius of effect before the virus is killed. That is assuming its a goldilocks planet with an atmosphere where the bio-constructs arent using a form of hazard suit against the environment. Digging in is a great idea, but if the enemy captures the surface they can dig themselves and avoid your defences. Any additions? $\endgroup$ – Demigan Jun 13 '18 at 14:58
  • $\begingroup$ You've changed the question details way too much for my answer to have much relevance now. $\endgroup$ – Serenical Jun 14 '18 at 8:45
  • $\begingroup$ my question details havent changed much. I've put in more constraints to make the possibilities less broad such as what kibd of tech you can expect, and I'm still focussing on how the defenders stay alive/repel the invasion rather than how they would be killed. $\endgroup$ – Demigan Jun 14 '18 at 9:44
  • $\begingroup$ Yeah, the technology constraints are what have changed. They're going to be what's most important. Defending against a cave man is a lot different than a modern soldier, and there's going to be just as big a gap between a modern soldier and your people. I assumed an unadvanced native population, which you have changed. $\endgroup$ – Serenical Jun 14 '18 at 10:09
  • $\begingroup$ Plus there was zero magic before. $\endgroup$ – Serenical Jun 14 '18 at 10:10
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Since we are essentially speaking of a fully realized spacefaring civilization, one question which comes to mind is why even bother settling on the surface of a planet? When you take that approach, the answer was already written by Freeman Dyson, who suggested the ideal setting for space dwelling life would be genetically engineered trees growing from the core of a comet.

enter image description here

Dyson tree orbiting a planet

This has huge advantages over growing a city on a planet.

  1. There are trillions of comets and small asteroids in Solar Systems, so launching seeds into the Oort cloud means you can simply provide an overwhelming number of targets any potential enemy has to deal with.

  2. You have access to bountiful solar energy, unlike on a planetary surface. The tree itself is an energy collector, and leaves can be genetically modified to act as solar sails, providing inexpensive, if slow transport throughout the system.

  3. With proper genetic engineering, the tree is recursive. If a branch is shot off, the people living aboard might still survive, keeping a symbiotic relationship with the branch to recycle their waste as nutrients for the tree, while the tree supplies oxygen and food. They might be able to last quite a while awaiting rescue.

  4. With trillions of trees, you can engage in all kinds of strategies, from playing "hide and seek" (which of the trillions of trees are actually occupied?), to mounting guerrilla wars against invaders, or using the trees as launch pads for high tech weaponry (invaders are simply overwhelmed by firepower coming at them from a complete sphere wherever they go in the system). Even simple coil guns launching at interplanetary velocity can deliver an amazing amount of kinetic energy at the target (and given the distances involved, you might be fending off a swarm of projectiles for months. The coil guns themselves can be repurposed mass drivers used to deliver cargo to and from the trees.

So remain in space, and use your biotechnology to fill the system with Dyson trees.

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  • $\begingroup$ This is basically how most of the planets were terraformed :). While there is space-living, I want to focus on planetary stories. I've updated the post to give better information. $\endgroup$ – Demigan Jun 13 '18 at 15:07
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The main worthwhile reasons to defend a city from ground attack are:

  • Protect the leadership (not relevant - population is bio constructs)
  • Protect non-combatants (not relevant - population is bio constructs)
  • Protect culturally-important sites (not relevant)
  • Preserve militarily significant resources (not among premises)
  • Defensive warfare - delay while marshaling resources to regain the initiative

Most of these reasons aren't relevant in this scenario, but one is: To tie down B forces while preparing a counterattack. This works even better if the city is a decoy or trap, and has no real strategic value to A after all.

Before you get to the city: Modern warfare is done in layers. Orbital weapons smash the landing ships before they arrive in orbit. Orbital and ground-based and air-based weapons smash the landing ships entering the atmosphere. Long-range energy- and kinetic- and chemical-weapon bombardment pummels the landing zones and assembly areas, etc.

The basic theory is that the enemy doesn't get any rest, doesn't get a chance to rearm, doesn't get a chance to refuel. An enemy vehicle without fuel or ammo is an easy kill, so B's resupply vehicles shuttling up to the main forces are a ripe target for A's hidden spotters and ambush forces.

To defeat the main forces, channel them into kill zones using natural and artificial obstacles, and concentrate fire upon them until whole units are dead.

Urban design for warfare has similar goals, but the methods differ as units get smaller. For large units, use channelized kill zones and ambushes to defeat vehicles and foot units. This means you need two street networks - the one B sees and uses, and the hidden underground or in-building network A uses for reinforcement, resupply, and observation.

The obvious street network that B uses should be interconnected and robust-looking...until B tries to use it and finds piles of rubble and other blocking obstacles. After the obstacle plan, only a few good routes should be left, and those should lead in the direction that A wants B to attack. Obstacles must always be overwatched by fire, lest B clear them.

Direct engagement is too costly; A's goal is to buy time. Use lots of mines, snipers, deadfalls, booby traps, mortars, decoys, noisemakers, and occasional counterattacks to keep B's forces wary, slow, hungry, stressed, and tired. A surprise every few feet, a couple in every room, and new ones stealthily placed into previously-cleared areas. Deny B's forces sleep, food, and resupply when possible to slow their advance even more. This means bare streets and bare rooms, too narrow for vehicles and too winding for vehicles to build up momentum, without cover or concealment...but with lots of easily-replaced bricks or cobblestones to hide surprises and observation devices behind.

The few streets that are (just) wide enough for vehicles should have long slightly-curved blocks so you can trap a whole convoy by blowing the front and rear vehicles that cannot see each other.

Building heights should vary, with lots of spires and poles and nets strung between, and frequently changing, to confuse or prematurely detonate air-launched ordnance. The wild skyline also hides A's mortar teams and anti-aircraft missile teams.

Many buildings should have false walls and secret rooms to hide snipers, ambush teams, and caches. Many buildings along major streets (of both networks) should be unobtrusively wired and mined for demolition to trap whole enemy squads within, and honeycombed with observation devices to monitor rescue efforts so the rescuers can be mortared. Hidden communication and observation taps for A to coordinate securely should be on every floor of every building.

Use raids outside the city to degrade B's artillery, air power, and resupply. Chemical weapons are especially good for this. Don't destroy all of B's long-range firepower - A's defense must hold their attention. Remember, A wants B to think they are making slow but inevitable progress until the counterattack is ready.

Just as the city itself is ideally a decoy, set up decoy HQs, supply dumps, barracks, and other obvious targets for B to target, while the real ones are hidden and hardened. Assembly areas for A's counterattack should not be in the city, and should not be detectable under any circumstances by B's forces. That would spoil the surprise.

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  • $\begingroup$ Just to see if it can be improved. Historically you need about 3x or more forces than the defenders (which is a PLANET, damn). Wouldnt the attackers start with clearing the orbital defenses? The attackers arent going to ignore them, and the orbital defenses can be cleared with relatively little interference of the groundforces. For supplies, you have to drag 3x the forces on a planet so wouldnt supplies best be gotten from the surrounding planetoids (anything useful will also be in use by the defenders) or the planet during the invasion? $\endgroup$ – Demigan Jun 13 '18 at 19:28
  • $\begingroup$ The cities defenses, how effective would it be? If every city is build the same, no attacker is going to use the "normal" roads would they? Wouldnt they set up artillery and systematically bombard parts of the city? How would the buildings look like? Wouldnt major cities be targets for orbital strikes? Would the defenders be better off with clustered cities or chains and chains of smaller towns to spread forces and reduce the room for enemy staging area's? $\endgroup$ – Demigan Jun 13 '18 at 19:35
  • $\begingroup$ @Demigan Whoa, hang on. You asked a question specifically about urban design for defensive warfare, so that's what the answer is about. It's not fair to back up and start asking about other, broader defense strategies - there are lots of other Questions here about how to defend a planet when FTL technology is available. Honestly, if A lets B land at all, they have lost the initiative and seem well on their way to losing the war. If B lands before months of kinetic bombardment (asteroids are cheap) have reduced A's martial presence to an impotent shadow, then they are fools. $\endgroup$ – user535733 Jun 13 '18 at 20:00
  • $\begingroup$ I didnt specify urban warfare. I specified (or tried to) how an army of bio-constructs could prepare a world for habitation while maximising their chances of survival during an attack. Habitated area's are likely spots for fights as thats where many soldiers are and they are wonderful to hide formations from direct attacks, but they arent exclusive. $\endgroup$ – Demigan Jun 13 '18 at 21:07
  • $\begingroup$ as for months of asteroid bombardment, I'm asking how the defenders can protect themselves, so if you have a good idea to attack the planet I want to hear a good way to protect the defenders. Also the power requirements to launch asteroids to the planets surface within months is going to be massive and too large an asteroid might make the planet hard to inhabit. $\endgroup$ – Demigan Jun 13 '18 at 21:13
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Purpose of any enemy attack is to spread panic and, most important, to cripple the city's activities. You need to build a modular city, in sectors that can each back up the other, to minimize the effects of infrastructural damage.

Also, the most important supports to this city's existence could be built underground, so that the enemy should choose between razing a 'Lego' city or raze the planetary surface just to get at the energy systems, refuges, hospitals, etc. -thus nullifying the purpose of their attacks.

But, anyway, since we are talking about a longtime enemy bound to destroy this civilization, they must know enough about these bioconstructs as to create a 'cancer weapon' to kill them off. And even if they won't succeed to eradicate them all the first time, they will simply perfection it

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  • $\begingroup$ I've updated the post with more information. Could you explain the idea behind your "cancer weapon"? It sounds like you are thinking of either a giant radiation bomb, but the "can perfect it against constructs" sounds more like a virus-weapon. Which I think has lots of problems as it needs to survive the specific planet and any countermeasures (like a series of viruses and bacteria that live off other viruses and bacteria). $\endgroup$ – Demigan Jun 13 '18 at 15:14
  • $\begingroup$ You're right: I was thinking about a virus weapon, the cancer word was a slip. At first I had thought of a radiation bomb, but of course that would also ruin the conquest field. Plus, a virus bomb as the advantage of being improvable every time and designed to work only against certain designated species without involving the rest of the ecosystem $\endgroup$ – Valerio Pastore Jun 13 '18 at 15:23
  • $\begingroup$ And how would you try to protect against such attacks? I would think that most planets aren't in the Goldilocks zone and won't even have an atmosphere, so the virus would have to go through a bad/absent atmosphere and then through the protective suits/space suits the constructs are wearing, not to mention having to deal with competition of other viruses and bacteria (your body intentionally lets various bacteria live on the skin as they are harmless to the body but will kill potentially dangerous viruses and reduce the amount that enter the body). $\endgroup$ – Demigan Jun 13 '18 at 15:30
  • $\begingroup$ Well, of course this solution will work only with goldilock planets. $\endgroup$ – Valerio Pastore Jun 13 '18 at 17:47
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I'm thinking of a non sentient fungus of sorts that has as it expands is hollow on the inside and a second organism to grow inside the hollow space with the actual city. And the invading species terraforms in different stages.

  1. Launch several bioconstructs A which is dormant, into the planet covered in species B. B grows a giant blob of fungus as soon as it lands and is dozens of meters high and kilometers wide. It is hollow on the inside which also provides protection to A
  2. When B is matured activate bioconstruct A by launching a second wave with a chemical agent (C), specifically crafted to penetrate B so that A it can develop into a city within the hollow inside of B

Depending on what different types of chemicals are in the C compound used for that area A can develop as an industrial zone, urban zone, agricultural etc

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  • $\begingroup$ Welcome to worldbuilding.SE! When you have a moment, please take our tour and visit our help center to learn more about us! This is a good first post! Thanks! $\endgroup$ – JBH Jun 9 '18 at 21:14
  • $\begingroup$ See my answer to Shadowzee. $\endgroup$ – Demigan Jun 13 '18 at 15:19
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I'm going to copy something from a game I like, Starcraft and its called creep. In the game the zerg live off the creep which provides them with nutrients and spreads across a planet. The zerg travel from planet to planet because behind them is the Protoss who glass every planet they are on. Glassing is basically eradicating all life on the surface of the planet using high powered laser beams.

Your most important aspect is speed and numbers. You can't do much against FTL travel so you need to be able to spread quickly. Once on a planet, you need to cover enough of the surface that bombing the planet and keeping it habitable is impossible. You can also dig deep. There is only so far a bomb can go and you can't cover the world in craters without destroying it which buys you valuable time. Your creep simply needs to spread over the surface quickly then force a series of tunnels underground where your species can live or survive before they move on.

Secondly you need numbers to be able to spread over a large area and have a fighting chance of beating the FTL travel. In this case, your creep bio constructs would form into massive cannons which it uses to launch itself towards other planets. It also doubles up as an orbital defense and you can use it against ships, especially if you allow your bio construct to form in space, around satellites and craters. Even better, if you hit an enemy ship you can instantly start spreading across that, using its materials to build a new cannon.

Basically its a cycle of arrive, spread, multiple, leave.

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  • $\begingroup$ I truly love the idea behind a zerg/tyranid type of species (but with flora added ofcourse), the terraforming has actually been done by such an engineered group of species. But the main species I'm talking about dont use them. I've updated the main post for better info, see if you want to change/add to your answer. $\endgroup$ – Demigan Jun 13 '18 at 15:19

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