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Context : my fictional universe is set in a time period akin to the european Middle Ages/Renaissance. Think of the 1400s-1500s for reference.

In most aspects (political, social, industrial etc.) my fictional universe closely resembles real life 15th century Europe, with two important clarifications:

  1. Contemporary technology and metallurgy has allowed a given kingdom to develop steam-powered machines. All other innovations that can derive from this aside, steam power has in particular led to the development of steam-powered airships, that is vehicles closely resembling merchant ships or carracks but that are airborne using airbags/balloons. These airships are also used for war, i. e. for reconnaissance, transport, or bombarding targets.

EDIT: To clarify, the steam and airship technology is enough for the airships to be effectively airborne (in a fantasy setting), but still relatively primitive enough for the airship's to be slow, clumsy, and with a low flight ceiling (let's say about 200 meters high). Also, their carrying capacity isn't too great and they can only carry about 10 to 20 men or the equivalent of a small to middle-sized fishing boat in terms of merchandise. However, they can be produced in relatively high numbers and are not a rare sight on the battlefield. Also, rival nations have started developing airships on their own as well.

  1. Gun powder does not exist in my fictional world. Whether this is because it hasn't been invented yet or because the universe's rules do not allow for this specific chemical reaction, Humanity simply does not possess it. So the main projectile weapons in use are the traditional ballistic weapons such as ballistas, catapults, mangonels, trebuchets, (cross-)bows, etc.

Assuming whichever kingdom that invented airships first gained a decisive tactical advantage thanks to them, and used them successfully against traditionally-built European medieval castles (for example by bombarding the castle structure and defenders with stones/arrows/other, lighting the castle on fire from above, or airlift and then drop invaders within the castle walls), how would the architecture of castles evolve to adapt to the threat of an attack from the skies ?

If you wish to contribute, feel free to give suggestions for active defense (i.e. weapons to fight the airships) as well as passive, more structural defense (i.e. the structure of the castle itself). I also gladly take suggestions/examples from other styles of castle-building (Japanese, Middle-Eastern etc).

Edit 2.0 : Thanks a lot for all answers ! I realise my premise is not the most plausible and may thus appear convoluted and difficult to work with. Nevertheless, I really appreciate the answers that tried to make do with my set premise as well as the various reality checks other contributors have given (which may help me make my setting more plausible/realistic). Cheers !

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    $\begingroup$ I don't know how to feel about steampunk without gunpowder... But it's your world. Quite an interesting one. $\endgroup$ – user70311 Nov 19 at 17:50
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    $\begingroup$ A lot is going to depend on the specific aspects of the Air ships. Steam power relies on fairly heavy boilers, and ships are also very heavy. Take a look at a modern blimp and see how large the lifting body is compared to the payload. With middle age technology its is going to be a much worse ratio. A meaningful answer to your question will depend a lot on how maneuverable the ships are, how much they can lift, how large their crews are, how high they can go, and how long they can stay afloat, etc. You might want to flesh that out a bit to cut down on assumptions. $\endgroup$ – abestrange Nov 19 at 17:50
  • $\begingroup$ Good suggestion regarding the airships' specifications, thanks a lot. $\endgroup$ – Kaloyan Nov 19 at 17:56
  • $\begingroup$ Is the setup scientific or somewhat magical? Steam-powered dirigibles are hardly viable in real life. $\endgroup$ – Alexander Nov 19 at 18:06
  • $\begingroup$ The setup isn't meant to be strictly scientifically possible. Assume it's a fantasy/magical world setting. In our case the airships wouldn't be viable against competition but they happen to be the only airborne military weapon. $\endgroup$ – Kaloyan Nov 19 at 18:08

10 Answers 10

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Self-contradictory question is self-contradictory

  1. "My fictional universe is set in a time period akin to the european Middle Ages/Renaissance. Think of the 1400s-1500s for reference."

  2. "Gun powder does not exist in my fictional world."

  3. "Technology and metallurgy ha[ve] allowed a given kingdom to develop steam-powered machines. All other innovations that can derive from this aside, steam power has in particular led to the development of steam-powered airships."

That world has nothing in common with the western European late Middle Ages or Renaissance periods. Or else, if it is indeed similar with the late western European medieval or Renaissance world, then it does not have practical airships.

Because:

  1. In a world without gunpowder there are no fortifications resembling late western European medieval or Renaissance fortifications, because late western European medieval and Renaissance fortifications were developed and designed to resist siege cannon and infantry with firearms.

    Krak des Chevaliers

    This is a pre-gunpower fortified castle, specifically Krak des Chevaliers, built in the 12th century in the County of Tripoli by the Knights Hospitaller. Note the nice regular shape, and those nice vertical curtain walls. Such shape and such walls would have made it easy prey to an army with gunpowder weapons. Photograph by Bernard Gagnon, available on Wikimedia under the GNU Free Documentation License, Version 1.2 or later.
     

    Fort Bourtange

    Fort Bourtange

    This is a Renascentist fortification, specifically Fort Bourtange, built in the the 16th century in Netherlands by William the Silent. Note the complicated star shape and squat sloped earthworks. The complicated star shape is designed to provide interlocking fields of fire to the defenders, so that the attackers have no safe area near the fortification. The sloped earthworks are designed to absorb cannon ball hits. Top, plan from 1657; public domain, available on Wikimedia. Bottom, aerial view, our days; picture by user Hannes, available on Wikimedia under the GNU Free Documentation License, Version 1.2 or later.
     

  2. There is an indestructible link between technological development and social development, or, as we would put in Marxist historical and dialectical materialism terms, between the economic base and the ideologic superstructure, or between the social existence and social conscience. You cannot have a medieval or Renaissance society with steam engines and airships.

    To have steam engines and airships you need a vast horizontal industrial base, with vast armies of relatively well-educated workers, who can read and write (so that they can operate the advanced machinery), who work for good wages, who must be fed, clothed and entertained by an even vaster army of service providers. You must have a small army of engineers to design your machines to make the machines to make the machines which make the engines, and the gas cells, and the cloth, and the lifting gas, and so on. To sustain this vast armies of economically active workers and service providers you must have a lively economy, with swift trade networks and pervasive financial services. That is not the late medieval or Renaissance world. It just isn't, in the same way that modern China with its enormous industry is not Mao's China.

  3. Ah, and how would it be credible that the people in the proposed world have advanced steam engines (no, primitive steam engines most definitely don't work in airships) and advanced metallurgy and advanced chemistry but don't have modern-ish explosives? In the real world, guncotton (a.k.a. nitrocellulose) was discovered in 1846, when airships were still half a century in the future, and TNT was discovered in 1863.

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    $\begingroup$ Thank you for the reality check. I did know indeed know about most historical facts that you pointed out, but I hadn't realised the necessity to correlate them to my setting for the latter to be plausible in the first place. I will try to reconsider my narrative based on your observations. Cheers ! $\endgroup$ – Kaloyan Nov 20 at 8:58
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    $\begingroup$ @Kaloyan Also, even if you forbit gunpowder's chemistry, there are other types of explosives. Pure hydrogen peroxide was first obtained in 1894 and acetone was first produced by alchemists during the late Middle Ages via the dry distillation of metal acetates - boom, you can make acetone peroxide, which is such a terrifying explosive even terrorists usually avoid it. But if you synthetised it right before takeoff and drop it on enemy city, it's going to be extremely destructive. $\endgroup$ – Tomáš Zato - Reinstate Monica Nov 20 at 13:35
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The good news is that without gunpowder and its derivatives, there's a limit to how much damage your airships could do. They don't have bombs or rapid-fire guns, and while they could drop incendiaries (like flaming oil) or just toss heavy objects over the side, they're severely limited in how much of either they could carry. Their main threat is probably troops dropped from above. Bows, crossbows, slings etc. would be a threat to exposed defenders but probably not to the structure itself. In short, while they would be valuable support, they're not going to replace a conventional army.

To mitigate damage, you'll want to avoid exposing too many people to airborne sharpshooters, and closing off potential points of access from above. Use tall, steeply-angled roofs wherever possible, and roof off the outer walls and galleries. Make sure the roofs are sturdy and well-sealed, and resistant to incendiary attack. Minimize the number of open courtyards, keep them away from vital areas, and make sure you have a plan to seal them off if attackers seize them. Avoid features like flat roofs or open-decked walls and towers where your enemy could congregate. You might consider where the enemy is likely to end up if they try jumping on your roofs anyway, and arrange spikes where appropriate, or just make sure that if they fall, they fall outside the castle.

When an attacker is spotted, your priority will be to get everyone under cover - at the very least under a roof, but preferably away from any courtyards or other locations that would make good landing points. Ensure that any troops that do land are isolated and can't attack your defenses from behind or link up with other attackers. Counterattack would be an option but I think it's wiser to focus on your enemy's main force first, then mop up the balloon corps if they haven't already taken the hint.

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  • $\begingroup$ Very nice, comprehensive answer. Gave me a lot to go on from. Many thanks ! $\endgroup$ – Kaloyan Nov 20 at 9:09
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    $\begingroup$ Note that even without gunpowder, there's still a way to make cannons etc: steam power! Steam guns were used during the American Civil War, with one design able to fire four rounds per second. $\endgroup$ – Skyler Nov 20 at 14:58
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    $\begingroup$ @Skyler Steam power is great, but it doesn't tend to be compact or lightweight - especially when you take into account all the fuel and water. $\endgroup$ – Cadence Nov 20 at 15:03
  • $\begingroup$ No spikes in the courtyards - you have them for a purpose, and spikes will just make them less useful. Just enough netting that anybody coming down will be entangled - just for the hope of some enemy nobleman would be foolish enough to try to lead such an attack (16-year-old noblemen with a fiery temper wouldn't be that rare in 1500, and these would be very valuable). i.e. yeah, steep roofs and securing the open spaces would be very much it. $\endgroup$ – toolforger Nov 22 at 13:29
  • $\begingroup$ @toolforger I was imagining spikes for when you need to join two roofs (because it's not convenient to have one 30-story spire in your castle) and there isn't a convenient way to channel people outside. Indeed, nets or just being able to seal off the courtyard and trap people there would be ideal for harvesting ransom. $\endgroup$ – Cadence Nov 22 at 21:40
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Air ships would not be worth much offensively.

Sun Tsu: "the worst policy of all is to besiege walled cities". https://suntzusaid.com/book/3

But if you gotta - consider: the walls are thick. Knocking them down is going to take repeated action - a ram, or many stones thrown, or cannonballs. If you are on the ground you can scavenge stuff to throw and you can get leverage for your ram against the ground. Up in the air neither is true. You have to carry aloft anything you are going to drop, at great expense.

The people in the castle will see you coming and hide. Your dropped boulder might make a hole in a roof. OK. Then you have to go back for another boulder.

But what about flaming liquids! You could drop those and burn everything up.

People in a castle who know that blimps with flaming liquids might come will not build out of straw. They will build the exterior out of nonflammable materials.

The roofs will already be steeply sloped if there is snow in winter. Roofs will be made of metal or slate or tile because those make great roofs. You will not allow wooden or straw roofs within the castle walls. Flammable liquids will burn on the roofs, or on the ground, and go out.

The offense balloon could have attackers rappel down ropes. How many of them can come at once? You can only fit so many people in a balloon. If you have a lot of balloons and you are keeping the defenders busy with your land forces maybe some could get in. The problem is balloons are not Trojan horses. They are obviously bad. The defenders inside the walls will notice the balloons and kill the intruders as they come down the ropes.


Defensively, however, balloons rock hard. My scheme for the siegebreaker balloon.

https://www.halfbakery.com/idea/Siegebreaker_20balloon#1269461429

The balloon would be tethered inside the walls. Charcoal for the hot air / ammo / sandwiches could be sent up to the crew via pulleys. Even a powerful catapult set up outside to assault the walls would be within range of the archers in the balloon.

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  • $\begingroup$ Very interesting twist on the possible military uses for a balloon. Many thanks ! $\endgroup$ – Kaloyan Nov 20 at 9:07
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First defenses coming to mind are passive ones, such as

  • smoke curtains, to prevent the airships from actually seeing their targets
  • fire updraft, related to the above, a large fire can cause a strong updraft which could disturb airships
  • anti-air crossbows, which should reach the flight height of the ships, possibly with incendiary arrows
  • barring nets, kept in position by hot air balloons (this is applicable only if the defendants understand the basic principle of hot air navigation)
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As the saying goes, fight fire with fire.

Your castles will want to develop their own airships for defense. If an enemy fleet can park over your castle and drop firebombs from outside of bow range, you're pretty much done. Making fireproof structures is really expensive, and while your Castle might be solid stone (on the outside), the village surrounding and supporting your castle will be quite flammable. Most sieges are about attrition, and continually razing the farms and hovels of the field hands would hurt.

Materials available to this period for the lifting bodies (even handwaved a bit to make this work at all) will be relatively frail and, more likely than not, flammable themselves.

An ideal defense would be your own "Hot air balloons" that have no heavy navigation equipment and are simply tethered, and can therefore achieve a higher altitude. The defensive airships could use their altitude advantage to fire flaming arrows at approaching ships to take them out / keep them at bay.

In direct combat between equal airships, the priority will be damaging the airships lifting bodies and forcing them to the ground. So flaming projectiles and "grapeshot" rounds will be your most effective weapons. Keep in mind how the cabin part of modern blimps hangs minutely under the massive lifting body though, you wouldn't be able to shoot at much of a vertical angle without hitting your own lifting body.

If you can figure a way to mount archers on top of the lifting body, you'll gain a substantial range advantage over the enemies, however.

Defenses on the ground will be sturdier roofs, less exposed flammable supports, ballistas on towers, and smoke screens to make it harder to be targeted.

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  • $\begingroup$ I don't think a 200m ceiling is out of bow range. $\endgroup$ – Martin Bonner supports Monica Nov 20 at 17:06
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    $\begingroup$ At 200m height, an arrow won't have much punch. Bowmen on airships, now those will fire arrows that have a height and range advantage - so a defensive line of airships should work. However, you need as many troops in the defensive airships as the attacker, so this approach isn't leveraging the advantage of the defender - being able to hold off a large army with a small defending force. (If the air troops cannot be put up en masse, then castles are still a force multiplier for the defender, though somewhat reduced.) $\endgroup$ – toolforger Nov 21 at 17:22
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The real issue is common airships would render the very idea of castles obsolete.

Castles and other fortifications were not just built randomly, but to defend important places like transportation routes or nexus points. So long as ground transport predominated, castles and other fortifications were of great importance. Even after the development of gunpowder and effective artillery, fortresses simply underwent an evolution of their own, from bastion forts, to "Third System" forts surrounding the American coast before the Civil War to the Maginot Line built prior to WWII.

Airborne and airmobile forces (and the increasing independence of forces using cross country mobility to reduce dependance on road, rail and canal networks have reduced the importance of permanent fortifications, and of course bombing and nuclear weapons also have made fixed fortifications far more vulnerable.

If airships are common as suggested by the OP, then the forces which use airships are effectively free from their dependence on roads, canals and rivers. Raiding forces, or even expeditionary forces can be delivered "behind" enemy lines without any reference to the fortifications, and supplies can be delivered without much chance of interruption by forces sallying from the fortress.

The main issue with air portable forces is they are essentially light infantry, and very dependent on the continuing delivery of supplies by air. Failure to do so will result in disaster, such as the British Parachute Division at Arnhem, or the French at Dein Bein Phu. This requires a great deal of thinking and planning by the users of air mobility, essentially finding prime points where air delivery can casue maximum disruption in the enemy rear, and creating conditions where the air assaults leave the enemy spread out and unable to concentrate against the main thrust (or even decide where the main thrust actually is).

So the main effect isn't going to be so much about rebuilding castles and forts to repel air attack. Rather it is going to mean the dispersal of forces to cover larger areas against airmobile troops, and the downgrading of large, elaborate forts to distributed barracks and strong points. The key change to warfare will be the discovery, isolation and destruction of airmobile troops before they can be reinforced or resupplied. Cavalry and quick firing "artillery" weapons like crossbows or ballista will become much more important as a means of countering airmobile forces, and light units like Rangers or Jagers will also become much more important as well.

A secondary consideration will be disguising or hiding important facilities from air observation and attack. Near the end of WWII, German industry was dispersed among small "huts" hidden in the forest, yet despite this aircraft production actually increased in 1944! Other examples are tunnel networks that the Vietnamese forces dug to prevent observation and air bombardment by French and American forces during the Indo China wars. More modern examples include siting facilities among civilian populations and in "protected" sites like hospitals and schools. How far along this process goes depends on how much airpower comes to dominate the battlefield.

So old forts may become abandoned or downgraded (much like many Bastion forts are now the centre pieces of modern cities), while military forces spread out and become lighter and more mobile to deal with incursions by enemy airmobile forces.

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    $\begingroup$ I feel like this answer runs quite a bit further ahead than is supported by the OP's description of the airship's limitations. They're only carrying 10-20 men, and not very quickly at that. Additionally, 200 meters of height isn't enough to pass unnoticed and/or unmolested by troops on the ground. Even for the purpose of inserting small groups of troops behind enemy lines, you can do that MORE effectively on foot or on horseback than you can using something that small and slow. $\endgroup$ – Morris The Cat Nov 19 at 20:56
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    $\begingroup$ The OP stipulates that airships are common, so while a single airship by itself isn't an issue, there is the ability to mass and deliver a fairly significant force, or create a useful logistical tail not tethered to roads or rivers. It is the ability of large numbers of airships which renders large fixed fortifications obsolete $\endgroup$ – Thucydides Nov 20 at 5:50
  • $\begingroup$ Thanks for the very elaborate answer. You took my premise and evolved it into bigger, strategic-level considerations, which is precisely (one of) the type(s) of answer I was looking for. Cheers ! $\endgroup$ – Kaloyan Nov 20 at 8:56
  • $\begingroup$ Your answer has an incorrect assumption: That air transport obsoletes land transport. This simply is not true given the constraints stated in the question (and it is not even true today even though air transport has become much more powerful). There's civilian transport, but even military transport, given the question's limitations, is not going to be useful for a full-scale invasion (but good enough for quick raids, so now border castles aren't sufficient anymore, a defender has to fortify a much wider border zone). $\endgroup$ – toolforger Nov 20 at 15:49
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If the flight ceiling is 200m, I would just build my castles on higher ground. At 200m, the airships cannot even get above your castle. At 120m or so, your archers can easily hit the balloons on an airship.

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  • $\begingroup$ I believe the idea was "200 meters above ground", not "200 meters above sea level". (The former would apply to hot air, the latter to hydrogen, btw.) But yeah the question is ambiguous about that. $\endgroup$ – toolforger Nov 21 at 17:18
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You just needs artillery that can shoot up by 200 meters.

This is actually available: The trebuchet. See https://en.wikipedia.org/wiki/Trebuchet#Reconstructed_counterweight_trebuchets for a rough idea what these things can do; since the airships are going to be pretty fragile, just have each trebuchet shoot 10 kg of bricks, and a single brick hit will take the airship down.

Trebuchets have a few limitations: They are expensive, they require a minute or so to reload, they are heavy (your towers and walls have a weight limit), they are hard to predict.
On the plus side, they have excellent repeatability. So trebuchet engineers will just fire test shots, put notches on the trebuchet so they can repeat that shot, and record the flight path. The real challenge will be to have observers that can quickly and accurately observe the view angles to an incoming airship; afterwards, it a question of having quick communication from observers to trebuchet positions, and a trebuchet overseer who knows what trebuchet is known to shoot through that point with what prerecorded settings. Plus enough experience to adjust for airship movement during the delay between observation and actual shot.

For the attacker, this means your airship have become an attrition weapon.
Which means you can't send your elite troops, and even normal troops will have to be forced to board those airships (a battle loss rate of 10% was considered atrocious before the invention of cannons).
Which means that even an insufficient set of trebuchets can be enough to deter an air attack.

Airships will have to get in in moonless, overcast nights.
Which places airship crews into a submarine-like state of mind: If you're spotted you are dead, so hush up everyone, no lights, and just pray to your god(s).
Of course defenders know that they are at risk, so they will try to light the sky. Send up balloons with bright fires, for example.

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  • $\begingroup$ Fires with curved mirrors might provide primitive spot lights that potentially could work better than out of control fire balloons hovering over your people's farm lands... $\endgroup$ – Muuski Nov 20 at 16:42
  • $\begingroup$ @Muuski you'd have to know in what direction to direct the mirror. But maybe a light source in the air is a bad idea anyway - anybody looking up will have a harder time to see those airships that are in the dark areas between the lights. $\endgroup$ – toolforger Nov 20 at 17:17
  • $\begingroup$ Actually, it would probably look like a lighthouse, with the light sweeping out over the sky and sentries keeping watch to see if anything lights up. More sentries on the side the wind is blowing from. $\endgroup$ – Muuski Nov 20 at 17:24
  • $\begingroup$ I was going to also suggest trebuchets as well as other siege engines. Ballista and catapults could be modified to shoot more vertically, and if pointed externally, would still have the advantage of damaging a ground assault as the shot material falls down. You could also design a load that breaks up at a certain point, allowing for a multitude of flying debris, such as a tied bundle of spears that comes apart due to a fire burning the twine holding it together, for the ballista anyway. Trebuchet and catapult often used smaller projectiles, rather than a single rock. $\endgroup$ – computercarguy Nov 20 at 17:37
  • $\begingroup$ @Muuski The problem with mirrors is that around 1500, they were too expensive. However, keeping bright fires lit would probably be enough to make airships at, say, 400 meters visible. You'd probably put a barrier between fires and spotters so the spotters have at least some night vision. $\endgroup$ – toolforger Nov 21 at 8:54
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In the time of wooden sailing ships fire ships were effective defensive measures. You now have likely flammable and fragile airships, use the equivalent of the fire ship, a fire blimp or fire mine if you will.

How might this work?
Off the top of my head you could go as simple as taking one of the airbags that would be used to float an airship and attaching outward facing blades to it with the hopes that if launched from directly underneath an enemy airship the blades would tear holes in their airbags once it rose high enough. This would make it immensely cheaper to produce than an airship.

Another idea would be to create a sort of basket on top of the airbag to hold a fire and a counterweight on the bottom to ensure that the fire basket stayed on top again with the hope that if deployed underneath an enemy airship it would rise and cause fire damage to the structure or airbags.

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    $\begingroup$ And then you have a (possibly on fire) airship crashing on your nice castle? $\endgroup$ – Rafalon Nov 20 at 14:28
  • $\begingroup$ @Rafalon ideally you would deploy these fire mines before an enemy parked their airship over your castle $\endgroup$ – BKlassen Nov 20 at 16:19
  • $\begingroup$ Fire ships work well because you can set up a line of them and the incoming fleet will have no way to evade them. For 3D situations, you have to put up a whole wall of fire blimps, which is much harder to set up and pretty difficult to maintain or control. $\endgroup$ – toolforger Nov 21 at 17:19
  • $\begingroup$ @toolforger not true, you don't have to target an altitude since the airships are slow to maneuver and the fire mines should rise faster and higher than the airships are capable of due to the mine having less mass. $\endgroup$ – BKlassen Nov 21 at 17:26
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As already said, without gunpowder or other explosives the attack value of airships will be low. To smash walls an onager, a ballista or a similar war machine would be more effective. But airships would be great for reconaissance or to drop command troops resp. assassins - but they would have to come in low for the drop.

Nets (spanned between towers) could provide passive defense (esp. if coated with shards), active defense would be arrows shot either by archers or by catapults. Even slingshots could effective if used by trained warriors.

If the airships had something like greek fire, now that would be a problem for the defenders. But it would make the airships even more vulnerable, too.

But air reconaissance could change the tactics in the open field. Troops would seek shelter in woods, buildings and so on, wherever possible; they wouldn't want to be counted by the enemy. Tight formations in battle could be prone to air attacks even with dumb stone bombs and so on.

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