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How would avians build a defensive structure? What would be the equivalent of walls, a drawbridge, a dungeon, etc.

If two avian / arial cultures were at war, say birds vs. dragonflies, how and where would they construct their defensive strongholds?

Would a siege be possible? Siege engines?

A human stronghold typically requires a wall or moat to slow the attacker, giving the archers a chance to cut them down. Could there be any equivalent for an arial species?

Assume mixed level technology, medieval level weaponry bows, bladed weapons, wooden structures, etc, with some advanced materials.

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Castles built to defend Avians from other Avian races would likely be built below ground. Either in cliff sides or in mountain crevices. However, these natural terrain formations will not always be options. Depending on the terrain, the Avians may build their fortifications as great shafts cut through the earth into bedrock. Lined with either cut stone or the existing smoothed rock, these holes would have shafts leading into the inhabited parts of the fortification. Super Colliding Super Collider!

Attackers would be forced to dive into a deep hole, ringed with firing loops manned by archers (or gunners) and attempt to breach one of the side passages recessed into the shaft walls. This would be a nearly insurmountable obstacle, and even a successful assault would entail incredible loss of life on the part of the attackers.

Sieges would be the natural solution to this design. As would attempting to breach the fortress via tunnel. Any fortress of this type would have to be designed with the understanding that the enemy may tunnel in from above or the side, and be laid out accordingly.

The defenders, on the other hand, will likely have sally ports scattered around the surrounding countryside, accessed by tunnels from the main fortress. These would allow the defenders to harry the besieging army and make contact with the outside.

Remember that many of these suggestions become immaterial if the Avians must also contend with land-bound races. In that case, the traditional "Tower on a Mountain" design becomes the most practical for the sheer difficulty of assailing it from the ground as well as its good command of the surrounding airspace. Castle in Slovakia

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  • $\begingroup$ As an addition to this you could build a moat by selecting a wind funnel fed tunnel crossing your main entrance with an artifical hazard on the leeward side so when there is not some sort of bridge extended it is extremely difficult to safely cross. $\endgroup$ – Myles Oct 22 '14 at 18:09
  • $\begingroup$ A hollow mountain nicely solves the issue of creating a structure with walls on all sides. It would be hard to attack without explosives. $\endgroup$ – superluminary Oct 22 '14 at 18:17
  • $\begingroup$ @myles, like taught razor wire for example. $\endgroup$ – superluminary Oct 22 '14 at 18:18
  • $\begingroup$ @superluminary That's true. Several other people had addressed mountains, however, and I wanted to try something different. $\endgroup$ – Danny Reagan Oct 22 '14 at 18:18
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    $\begingroup$ @DannyReagan - How about drowning in smoke? Easily reversible, though smelly. $\endgroup$ – superluminary Oct 23 '14 at 10:57
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This question is very difficult to answer without knowing more details: technologicl levels/capabilities (do they have projectile weapons? How fast/dangerous/speed-of-firy are they?) Engineering levels/capabilities? Aerobatic characteristics of the species? (e.g. slow flying birds can be defended against by small movable walls, ala ping-pong paddle blocking).

A couple of generic points can be made (some shamelessly stolen from Bean of Orson Scott Card's Ender's Game series):

  • In true 3-d, far forts and in general perimeter defenses are practically meaningless outside the effective coverage range of ranged weapons.

    Reason: Surface area to be covered increases as a square of distance; the volume as a cube of distance.

    For the same reason, sieges are far less effective without VERY good (fast rate of fire, auto-aiming, fast-to-aim) ranged weapons able to cover TONS of area/volume. That's why most spaceship SciFi has point defense lasers.

  • As with regular 2-d surface warfare, tactical and strategic use of terrain as defense is critical (even more so).

    • Your avian species, presumably, still have planet surface.

      So building a castle in a crevice of a mountain, so you're protected by earth below, and 2 sides of the mountain from the sides, is a major advantage.

    • Not all 2D stuff is applicable. "High Ground" becomes LESS defensible instead of more defensible, for obvious geometric and physics reasons.

      • Picking a windy area benefits the defenders against avian species, however, in a way that picking higher ground helps pedestrian defenders due to gravity.
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  • $\begingroup$ If it is windy, with a prevailing wind, you really want to make sure you control the upwind areas or it will allow blitz attacks to close extremely quickly, giving your defenders extremely limited time to prepare. $\endgroup$ – Dent7777 Jul 12 '18 at 0:51
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I would think that it would not look like any sort of castle that we're used to seeing. Birds have to think in three dimensions because they can move in three dimensions - as can their opponents. There's no sense in building a ring-shaped castle when your enemy can simply fly over the top and land right on you.

So what shape would it be? I'd assume a sphere. Spheres have no edges or corners, and so it would be hard for attacking birds to identify its orientation. It would also be hard to land on, because, ideally, its surface would be so flat that nobody could land on it without slipping. Coat it with, say, oil, and you've got a place that would be very hard to breach. This does leave the issue of how you're going to keep the whole thing in the air. You probably wouldn't be able to, so it could rest on a surface, like the ground. It would still be fully enclosed, though. So it would not be aerial, because any bits intended to keep it in the air would become easy targets.

  • Walls - These would, of course, be the outer edge of the sphere. These would probably be thick, like a conventional castle - many feet deep. They have to be strong and hard to break. Steel might be a good choice, as would iron. Advanced composites would have some merit if the society was duly advanced.
  • Drawbridge - Why would you want a drawbridge? There's no way you could create a three-dimensional moat, unless the sphere was at the bottom of an ocean (in which case everyone's in trouble). No moat means no drawbridge. Several doors would be necessary, though, complete with iron bars and all the other guarding regalia from the Middle Ages.
  • Dungeon - Just like a normal dungeon, I would think. Chuck an enemy in an enclosed room and lock them in, preferably with chains.
  • Keep - The castle keep is important; it's the central area where the king can be found. I'd embed a smaller sphere inside the big sphere, with some support columns between the two. You wouldn't need bridges because these things can fly.
  • Defenses - These depend on just how advanced these birds are. Obviously, guns can make a big difference over bows and arrows. Either way, though, expect to see versions of crenelations. Also, catapults would be difficult because there wouldn't be an open top to launch them off of.
  • "Moat" - You can't have a castle without a moat or other obstacle, but, as superluminary pointed out to me, there's not a whole lot of obstacles you can put around a large sphere. So we'll have to be creative. What would deter a flock of large, angry birds attacking a spherical castle containing other angry birds? Well, barbed wire is an idea. Let's have a whole bunch of large columns stick out at regular intervals along the sphere. Now string webs of barbed wire between them, so that the only gaps are smaller than a bird. This would take a lot or barbed wire and an elaborate system of support structures, but it would make it impossible for the enemy to get in while allowing defenders to safely fire weapons from arrowslit ports in the castle's sides - like this (viewed from the inside):

enter image description here Image originally from Wikipedia user Bkwillwm, licensed under the under the Creative Commons Attribution 2.0 Generic license.

So perhaps a shell of barbed wire around the castle could act as a moat defending against the attackers. You could also attach automatic or remote-controlled weapons on the columns to defend them from the attackers.

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  • $\begingroup$ It's a cool idea, how would you defend it? Would it be covered in multiple ports manned by archers? A human stronghold relies on placing some obstacle, a wall, moat or hill for example, that takes time to traverse between the archer and the attacker. While the attacker traverses the obstacle the defenders cut them down. With a flying enemy attacking a spherical structure it's hard to think of a suitable baffle. What would stop them coming in numbers, landing all over the stronghold, pulling the defenders out through one of the ports and then diving inside? $\endgroup$ – superluminary Oct 23 '14 at 10:12
  • $\begingroup$ @superluminary All good points. I addressed the moat issue in a recent edit. $\endgroup$ – HDE 226868 Oct 23 '14 at 15:47
  • $\begingroup$ A wire shell would provide a baffle without obstructing the field of fire overly. A similar structure could also guard the mouth of a cave. $\endgroup$ – superluminary Oct 23 '14 at 15:53
  • $\begingroup$ 2superluminary Do you agree with my assessment? What could I improve? $\endgroup$ – HDE 226868 Oct 23 '14 at 17:27
  • $\begingroup$ You said, "What would deter a flock of large, angry birds attacking a spherical castle containing other angry birds?" The answer, of course, is piggies! $\endgroup$ – Octopus Oct 23 '14 at 23:48
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The answer by HDE 226868 already provides some great ideas about how the fortress itself would be designed. I would like to write about another aspect: Location.

Medieval castles are built on mountains, because this makes them hard to reach. The access routes are few and narrow, making it impossible for attackers with superior number to attack all at once.

When transferring this logic to an aerial attack, the opposite becomes true: The amount of airspace needs to be limited as much as possible, so placing the castle on a mountain would be a weak location. The enemy could easily surround it and assault it from all sides at once, from above and from below.

It would be a better idea to place the castle at the foot of a mountain, because this reduces possible attack vectors. The defenders can concentrate their anti-air fire on a smaller part of the sky.

An even better location would be beneath an overhang, in a cenote or inside a large cave, because that way it could only be attacked from a single direction which might even be small enough to block with a gate.

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  • $\begingroup$ The foot of a mountain is sensible in terms of reducing the field of fire. $\endgroup$ – superluminary Oct 23 '14 at 10:23
  • $\begingroup$ Actually making them hard to reach was not the only reason to put castles on mountains. Being on a mountain also meant you had a wider field of view, and thus could spot attackers already at a larger distance, giving you more time to prepare. $\endgroup$ – celtschk Oct 24 '14 at 8:54
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Tons of good answers already, but I can't help myself but try to my hand too.

In terms of passive defense mechanisms, regardless of 2d/3d, you basically have to think of how you (or rather your avian species) can:

  • slow/fatigue the enemy
  • funnel the enemy
  • stop the enemy

These same things can happen far from the fortification, nearer the fortification, and even inside the fortification.

Below are a bunch of ideas (got carried away), but the point is really the above: Slow, funnel, stop.


As already mentioned, tunnelling or burrowing would be obvious solutions for an avian species (indeed many birds in our world burrow or tunnel). You're protected on most sides, and funnel an attacking force quite naturally.

One could also consider burrowing upwards. A cliff by the sea could easily have some caves worn by water. From there you might burrow upwards into the cave's ceiling, and (given time), you'd have an upside-down mine. It might be a pretty natural idea for an avian species, just like digging downwards is for us.

Natural obstacles

The trick, though, for any of those structures would probably be in generally slowing an advancing enemy when that enemy can fly. A human castle might be near a river - or even swampland - which is difficult to ford or cross. And it also make it easy to spot attackers.

It's harder to think of something that would do the same in an airspace. Giant trees or floating rocks might work if your world otherwise allows for such contrivances. But it would also hide attackers. Giant cobwebs would be neat, though. Or: Large panes of clear glass. Many a bird have fallen victim to that.

But there are some naturally occurring things that could hinder flight:

  • A smoking/ash-spewing volcano is basically a vertical river that'd be hard/dangerous to "cross". (And it's a pretty dramatic visual, if nothing else)
  • Hot springs or giant geysers producing a lot of steam might work too.
  • Natural wind tunnels, and places with vicious currents, like those between tall buildings or near mountains.
  • Really tall mountains. Low temperatures and thin air affect any living being. (See also: False sense of security. Hannibal crossed the Alps in winter, after all.)
  • An area with near-constant precipitation and/or fog making flight difficult. Swimming birds have feathers that repel water, but other birds aren't so lucky, and can't fly if they get soaked.
  • Conversely, an arid desert area might have a perpetual sandstorm - not something you'd want to fly through. Plains might have tornados and lightning storms.

None of these will stop an army, but it could wear it down, and make an ordered advance impossible.

Speaking of precipitation: The good ol' cave-hidden-behind-a-waterfall. The defenders could have a retractable "tunnel" (or just a big awning) that would part the waterfall from inside the cave. Once retracted, attackers would be flying into a moving wall consisting of a few tons of falling water. (Though if I were attacking, I'd think about diverting the water somewhere upriver, if possible. Doing so for something the size of Niagara Falls would be pretty difficult, though.)

Passive defenses

In terms of more purpose-made passive defenses, smoke is a good option if you've burrowed into the ground, because smoke rises. It could make the dive into a shaft even more difficult.

Any type of fire, with or without smoke, could also cause strong updrafts that might be considered "difficult terrain" for anyone flying.

A hole in the ground like that proposed by Danny Reagan could be ringed by fire at the top: Not only would you have to dive into a heavily defended shaft that you can't see clearly, you'd also have to pass through dense smoke and updrafts first, and avoid getting singed on the way in.

A really devious (and pretty Looney Toons'y) idea would be to camouflage the real entrance, and light a ring of fire somewhere else. Brave attackers would dive for the center... and faceplant (beakplant?) into solid rock.

A horizontal tunnel could be blocked with a literal trap, like you see under a sink: A place where the tunnel "bucks" up or down. A low part could be flooded with water. The defenders would also be locking themselves in, of course, but unlike a cave-in, it'd be reversible.

A really cruel idea: Heavier than air gasses. Perhaps your world has a natural spring of sulfur hexafluoride or something. Fill the trap with that, and make flight really difficult - maybe even asphyxiate attackers, drowning them in "air". Of course a similar thing could be used by the attackers if the fortification is lower than its entrance.

Defenders at the bottom of a shaft might try the opposite: Lighter than air gasses. Though you'd need a lot you could create pockets of "light air" where the attackers wings won't carry them.

Hydrogen balloons could work as floating mines. Poke one and it'd (somehow) ignite. The fireball, while maybe not deadly, could singe feathers and prevent flight.

Weaponry

As for actual weaponry, others have already mentioned that your defenders might want to use ranged weapons with large cones of fire (e.g. birdshot) or AA-weapons like flak cannons. It may not be necessary to kill if your defenders can simply make flight impossible. Slinging tar or pitch could also interfere with flight by gumming up plumage. It's like anti-personnel mines, if less visually horrific. Still, removing a creature's main means of transportation is a horrible thing, regardless of how you do it, but that's why it'd be effective.

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  • $\begingroup$ Great first post. A little wordy, but considering some of mine, I shouldn't judge! Pretty comprehensive, +1. $\endgroup$ – HDE 226868 Oct 23 '14 at 23:52
  • $\begingroup$ @HDE226868 Thanks! And yeah, wordy, I know... got carried away :) $\endgroup$ – Flambino Oct 23 '14 at 23:59
  • $\begingroup$ One thing you can do is to add in either a summary at the top/bottom of a "TL;DR" (Too Long; Didn't Read) at the top. $\endgroup$ – HDE 226868 Oct 24 '14 at 0:00
  • $\begingroup$ @HDE226868 But that would ruin the rambling nature of the thing :) But seriously, I'll probably let it rest a little and edit later with clear head (already noticed some typos). For now, it's basically a brainstorm, but I figure that's ok. Besides, the 3 points at the top is the real take-away already. $\endgroup$ – Flambino Oct 24 '14 at 0:06
  • $\begingroup$ @Flambino - awesome first post, I love the idea of a waterfall guarding the entrance, it would be practically impassable. $\endgroup$ – superluminary Oct 24 '14 at 10:37
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Once again cliff faces make a good defensive position. Hard to drop something from above and door ways can be blocked. Scatter weapons would be very useful on defense, either large flights of arrows or later tech using gun powder, shotguns with "buck shot" or cannon with 'grape shot' would be very effective since it covers an area and requires less accuracy.

Though with a cliff face you'd want to keep heavy enemy weapons out of range, if all they need to do is point at your cliff 'wall' and cause damage. Having good overhead protection becomes much more important with the ability to have more accurate projectiles dropped from on high. This might be very steep roofs to help deflect rocks. (assuming generally rocks small enough to be carried by one and still allowing them to actually fly) Using multiple avians to carry larger stones would require a lot more training to fly in synch and they would be a larger, slower target, so would be easier to defend from the attempted attack but harder to deflect a successful drop.

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I'd have difficulties seeing a castle in a traditional sense...In day to day life, most activity occurs outside the castle walls. It's when an enemy approaches that people relocate themselves in to the safety of the walls. In a "3d" world, walls don't do as much, you'd need a roof as well.

In the case of a bird or species that inhabits the ground (builds nests and other structures on land or in trees), I suspect a castle would be very unassuming...simply a cave entrance or hole in the ground. When an enemy comes, the majority of the population would enter the cave 'castle' to protect themselves from the flyers above. A drawbridge here is simply blocking the entrance from the inside.

Defensive 'turrets' as bunkers dug into the ground (possibly not even connected to the main castle' cave) with some capability of firing into the sky would likely be the best defence (scattered through the town/village, camouflaged when possible). Roman style artillery would be a good choice (ballista's, Onagers, Scorpio's).

In a completely ideal setting, the narrow entrance to the cave castle would open up into a large cavern...gives a narrow opening for the enemy to cram through while the defense is well spread out and focused on the narrow choke point that the invaders must enter.

An insect species might be a bit different. Bird-like still has a heavy ground component where it nests and lives on ground while not in flight...insects are different as they don't need this component and are more likely to build in the air. I wonder if a giant 'bees nest' structure would became the dominating 'castle' image.

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  • $\begingroup$ I like your point about different flying characteristics. A hovering species could effectively defend against a fast flying species with a suitable choice of location. Likewise a creature with the capacity to fold it's wings and enter a narrow gap could simply make the entrance to the fortress too narrow for the enemy $\endgroup$ – superluminary Oct 23 '14 at 10:22

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