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The question

How would a fortress built on a floating island look like? what kind of defensive emplacements and structures could be expected to be built for such a construction in order to defend it from attacks in the setting described below?

Context

The world is a gas giant planet about twice the size of earth, it is covered with floating landmasses at different heights, some in places where life as we know it can and has developed.

The fortress

The fortress was built to secure a nation's control over a strategically important resource and to prevent enemy raids and attacks on the local infrastructure.

The island

The island on top of which the fortress sits is a floating rock of about 400 x 400 square meters of irregularly shaped and plateaued terrain on top (some parts of the island being at different heights than others) and a thickness of 200 meters at the center, decreasing towards the sides.

There is no ground below the island, only clouds (the planet being a gas giant). The island is also static, meaning it doesn't naturally drift or fall unless a huge force is applied to it.

The island's structural integrity is solid enough for a network of tunnels to be build without risk of collapse and there is no risk of the island toppling over due to unbalance because of the mechanism that makes the island float.

There is, however, a risk of the island beginning to fall, should more than 1/3 the island's weight were put on top of it (about 25.000 metric tons with some quick calculations).

Technology

The technology is all over the place.

The world is currently undergoing an industrial revolution thanks to the discovery of new sources of iron, copper, tin and other metals, before this, metallurgy was limited due to the difficulty of access and collection of these metals.

The steam engine was never invented, instead, people rely on the strong and constant winds that blow past the islands to power the booming industry. Magically powered engines are also used, but in a much smaller scale.

Despite all this, flying machines have existed for quite a while in the form of hot air balloons, canvas gliders and canvas airplanes (metal airplanes being only a recent invention).

Firearms also exist in this world in the form of semi automatic rifles and canons powered by magic instead of gunpowder.

There is precision engineering and manufacture, so mechanical computers are not out of the question.

Magic

The mechanism by which islands float is similar to flux pinning, but people don't harness its power as they don't really understand it.

There is another type of magic related to elemental spirits. People can trap these spirits using specially designed containers to use them as batteries and power different machines and devices, before the spirits are completely depleted they can be recharged with their respective elements. This is done to power devices for which it may be cumbersome to carry a power-source or generator, such as airplanes or guns, powered with fire spirits.

Politics and warfare

With the exception of a few uncommonly large islands, most islands are ruled in their entirety by a single political entity, meaning that ground based warfare is pretty uncommon. Instead, most battles are fought with airplanes and airships.

The extent of an infantryman's work is dropping from aircraft to an island to secure structures and assert control over the population. But it is rarely, if ever, that they have to actually push the enemy lines. Their role is similar to that of a marine.

Recently it was discovered that below the clouds (the nether) there are islands covered in soil rich in metals. This soon led to a huge political conflict between many nations over the control of these nether islands.

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    $\begingroup$ You may want to spell out that the planet is a gas giant with several floating land masses that life has developed and is sustained on...I had to read this a couple time to figure out that this was the only possibility... $\endgroup$ – HighSage Jun 28 '16 at 3:44
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    $\begingroup$ Done, thanks for pointing it out, I hadn't noticed I'd left that piece of info out. $\endgroup$ – Miguel Bartelsman Jun 28 '16 at 4:32
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    $\begingroup$ 400 square meters or 400 meters square? That's quite a difference. From your load figures I guess 400 square meters, i.e. the rock is roughly 20 m by 20 m by 200 m ... $\endgroup$ – o.m. Jun 28 '16 at 4:40
  • $\begingroup$ Fixed, 400x400 meters. thanks! $\endgroup$ – Miguel Bartelsman Jun 28 '16 at 14:27
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If I were designing such a fortress, I would place ballista's around the island covering 360 degrees. As most of the transportation in your world relies on balloons, the easiest way to sink attacking air ships is to puncture their hulls. They could be modified to also fire ball-and-chains as well as spinning blades, designed to cut into and disable the canvas of gliders.

Depending on the mechanism for the air ships buoyancy (if they're filled with flammable gas), the arrows may contain oil and a fuse (similar idea to cannonballs). In this case, the arrows would be designed to latch onto the balloons rather than create the largest possible puncture, and wait for the fire to take the ship down.

Because the island is floating, defensive fortifications would have to account for attacks from anywhere around the fortress - including attacks from underneath the landmass. Ballistas could be in bubbles protruding from the floor of the island, with internal tunnels allowing access for soldiers to man them. I imagine them as similar to aircraft gunner ports, but with giant metal ballista and arrows instead of guns. It would also be useful to have simple rocks rigged to drop and the command of the fortress's defenders, ripping the canvas off unsuspecting gliders and popping air ships passing underneath.

All of the buildings should be mined into the island, with minimal entrances to the surface of the land. (In fact, it would be a good idea to place bombs all around the upward surface of the "ground.") The less that's visible to the air, the harder it is to take for an attacking force. It also creates choke points for troops attempting to breach the fortress. There would likely be minimal walls; as the real battle would happen in the air - not on the islands surface itself.

An airfield would likely be on the island, and it would make sense to booby trap it in the event that forces disabled the balista's and attempt to land on the fortress. Soaking the field in oil or utilizing your magic would make for a nice surprise.

Assuming the island doesn't lack resources, I'd also place tethered balloon scouting posts floating far above the island. The largest threat to such a fortress would be an air raid with bombs or massive amounts of weight intending to sink the land. As such, defenders manning said outposts could watch the skies above for possible attacks and signal defensive responses.

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On modern day Earth, a floating island used for military purposes is called an aircraft carrier. Prior to the development of reliable powered flight, floating military islands were called battleships or sometimes dreadnoughts. Before the ability to make large ships out of metal, wooden floating islands for military purposes were called "Line of Battle ships".

What I am getting at is the "floating island" part of the question is a McGuffin, what is important to consider is the dominant military technology of your setting. If it is a setting in the middle ages, then the floating island will be full of soldiers, catapults and probably fields to grow crops to feed everyone. Today it might only have some remote sensors because everyone can see the island and will target it with a nuclear weapon, so your military will not want to be sitting on a giant floating target.

An interesting middle ground might be a WWII era setting. A British "mad scientist" named Geoffrey Pyke developed a composite material consisting of @ 14% wood pulp mixed with water and frozen. His plan was to create a "pykrete" aircraft carrier the size of an iceberg which would be essentially unsinkable by any existing or projected German weapon, and could serve as a mobile airbase for 4 engined heavy bombers. Based on the description of your setting, a mobile "airbase" is what you are talking about, so we are looking at a giant aircraft carrier.

Pykrete aircraft carrier

There will be one or more flight decks to launch and recover aircraft, and depending opn your setting, a multitude of anti aircraft batteries to defend the airstrips and shoot down approaching enemy aircraft that slip past the combat air patrol. A modern aircraft carrier has missile launchers and the Close In Weapons System (CIWS) to deal with incoming missiles and aircraft, but you might have "flack" 88's or whatever fits your scenario best.

Oddly enough, the other possible inspiration might be the "Battlestar Galactica", since it is effectively an aircraft carrier in space, so there is a more 3 dimensional flavour to how they operate.

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It seems like this island is primarily a base for aircraft and is situated in/near an airstream utilized for travel and floats over another island mined for metal ore (but concealed within clouds). So the fortress needs to be able to control the airstream, deny the enemy from occupying the ground beneath it, and maintain a clear airspace above it to prevent bombardment.

Presumably the top of the island is mostly armored hangars and landing strips/pads for aircraft. All the people would live in bunkers. SO it would look less like a castle and more like a Japanese island from WW2. Everything critical is within the tunnels because it would easy to bombard the island from above. Below the islands "horizon", assuming it looks like an iceberg or something from Avatar, there would be tunnels leading to armored bunkers protruding from the sides like blisters.

If the island has the structural integrity to be heavily landscaped, the island may be carved into a shape that maximizes offensive capabilities (a giant octahedron with smooth upper contours to deflect falling bombs, reverse terraced lower portion so airframes can hang, ready to drop into action and allow for a clear field of fire straight down, big open hangers to allow for aircraft to land in enclosed bays, etc).

There has to be 360 degree (x, y, and Z axis) observation and anti-aircraft capability. If the enemy craft are relatively fragile dirigibles and the like, then high rate of fire/low penetration weapons should suffice. But if armored "sky battleships" are possible then there will have to be shielded bunkers containing heavier cannon. But the best defense is a good offense so constant "CAP" (combat air patrol) and multiple means of launching defensive aircraft would be the most effective means of accomplishing the mission.

Since the land below is concealed by clouds the only way to effectively target anything there would be to have predesignated target points that island based artillery could hit based on the directions from a ground based forward observer (this would require the island and the land below to be VERY static in position WRT each other, or the TPs would have to be constantly updated). A more fluid battle would require signaling like a flare launched up through the clouds to give the fortress something to aim at. Aircraft could obviously fly below the clouds and hit an invading army, so obviously this is why the fortress must be taken in the first place.

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To start off, I would suggest against the use of much metal in the construction as it is highly dense and would not go far with your weight limitations. You may look into whether there is any way that your civilization has the know-how to modify the island in order to give it more lift which could compensate for added weight.

Being an floating island, you will only be attacked from the sky which will require a based aerial combat team as well as a fast means of deployment. A runway may be necessary for landing, but your best bet for deployment of aerial units might be to have tunnels in the island itself with a 20 degree grade to them to allow units to pick up speed in a short distance before launching out into the air.

An aerial proximity mine field may also prove useful. Placing them on the edge of your boundary with a potentially hostile force will reduce their ability to strike with little notice as they would have to take their time to navigate through the field or go around. Ideally you would have scouts patrol the field and each would have a device onboard that would deactivate any mines within a certain range of them. This would allow your scouts to patrol with ease and safety and should they spot anything, they can get clear to send warnings leaving a trail of reactivating mines behind them (you would have to ensure these devices did not fall into the wrong hands though - or allow them to as a plot point).

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