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?
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 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 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).
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.
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.