Sky World, by its nature, offers some unusual transportation challenges and options. Travel between the islands is not easy. Despite having a type of wood that is naturally buoyant, you can not make a raft out of flotsam or dead fall wood. People require a good deal of wood to support them, plus a means of controlling where they go.
Travel between the floating bodies is generally by flying ship. Gliders can go to windward, but are slow and small. A very few ships use mages or magic to go to windward quickly. Such ships tend to be expensive passenger liners, military ships or specialized couriers.
Ships are mostly sail propelled with directional control provided by steerable parachute-type kites. Ships can maneuver from roughly 20 degrees off of perpendicular to fully down wind. There is no way to sail to windward. Commercial trade ships tend to be large and sail propelled. Such ships have to wait for seasonal wind changes to retrace their course, or sail in routes that shift between prevailing wind patterns. Ship designs fall into a few categories. There are zeppelins or box shaped types that use forward spinnaker type sails. They will land vertically if they land at all.
Tethered ships can accomplish travel between local bodies, i.e. islands that are only a few hundred yards apart. The ship is drawn back to its starting point via tether, plus the tether permits the ship to angle across the wind.
Travel on the floating bodies tends to be by foot, human or oxen drawn carts or rickshaw. The larger bodies will have horses. Humans or cattle are used for most plowing and farming tasks. Horses are too expensive for such limited use.
There do exist what might be called generation ships. These are extremely large sailing vessels that travel forever. Typical size is 1,200 to 1,500 feet long and 300 feet across and 200 feet in height. They will only travel down or across wind, never to windward. Their crews often live most, if not all of their lives on board. Such a ship might call once in a lifetime on any given island. All of the crewmembers are related to each other if of the same race. The crew will number often between 2,000 and 5,000. Such vessels will trade goods and ideas over long distances. They will usually stop only at the largest islands in a region. The vessel will spend a few months trading and then leave for another region maybe 1,000 to 5,000 miles away. Player characters may not come from a generation ship.
I am thinking of letting teleportation be the big transport method. Each island would have one or more points that allow people to teleport to another island.
Sky World is pre-gunpowder Dark Age, although magic makes a major difference. The availability of aerial units also has a major impact on both tactical and strategic planning. The lack of sulfur in large quantities makes gunpowder unavailable. Limited metals are also a major hindrance. Sufficient research could discover guncotton, dynamite and nitroglycerin.
Cavalry is almost unknown. Horses are very expensive to maintain on the relatively small landmasses of Sky World. Sword and shield armed Infantry is very common, albeit unit sizes are small. Pikes and spear phalanxes are known, but generally consist of untrained peasant levies. Archers, using wood tipped arrows, are used both on land and on aerial units. Tactically, battles consist of a mass of people throwing themselves at another mass of people. Trying to get pike or spear units to maneuver beyond a straight charge is unheard of. Close order drill of spear or pike units is a lost technology. The typical unit is a powerful or charismatic sword fighter plus a bunch of sword & shield followers. Individually, the people are great fighters. As a group, they have no real coordination or group tactics.
Aerial units add literally another dimension to combat. There are no true paratroopers, but the ability to land men and material behind normal lines add complications to both strategic and tactical issues. Holding the weather gage is important in the sky for ship to ship combat, but also for land combat. There are the no equivalent of aircraft carriers and marine assault ships.
Magic based units also exist. Mages can, and do, make flying carpets and broomsticks. Such aerial units can act like modern Earth fighter jets and to lessor extent bombers. Small aerial units act as transportation units, but also as combat units. Weapons are usually swords or magical wands. Large aerial units will mount wands, but will also have mages and conventional weapons. Mages will have access to the usual spells such as sleep, fireball, lightning bolt and telekinetic attacks. Conventional weapons on large ships will be catapults, bows, and similar siege equipment. Boarding parties are common, consisting of both conventionally and magically armed troops.
Aerial units add significant complications to strategic planning considering the overall technology level. Any city-state or landmass is open to aerial invasion from neighbors' nations or even from pirates. A very few large countries are able to afford war ships that can travel upwind, which gives them a major role in world affairs. Upwind countries have a significant advantage when attacking their neighbors. It is not an overwhelming advantage, but it does have some strategic effect. This is similar to Pournelle's essay about the types of faster than light drives have on science fiction space empires.
Ships can appear on the horizon and attack any island with little warning. Small, scattered, weak settlements would be prone to raiding. Larger settlements are tougher to raid since there is sufficient manpower to put up a good fight.
Fortifications reflect the existence of aerial units. Medieval European style castles do not exist. European castles are designed to protect against land based troops only. Sky World fortifications tend to be flatter and have bunkers and limited anti-air protection. The flatter structures permit moving troops to intercept glider or broomstick or magic carpet landings. There are still walls such as American Army western forts to protect against conventional troop attacks. Underground shelters that require intensive manpower to clear were common for pure military sites, but the current economy and military technology does not encourage new ones. Such ancient underground fortifications provide significant sources of adventure.
Siege engines are basically unknown. The ancients had them, but the art has been lost, and is of little use now. Raiders do not have the time to construct such engines, nor to carry them from island to island. Battles tend to be decisive with regards to capturing and island. Siege warfare is rare. Magical aerial units make it to easy to lift or prevent a siege. It would require control of both the lands around a city and the skies over the city to make an effective siege. The limited fighting between city-states makes large armies uncommon; the smaller lords do not have the resources for extended multi-month battles or sieges. Barbarian raiders, while few now, are only interested in hit and run raids for loot and slaves.
There are few, if any countries that can afford to station a garrison, or re-settle every abandoned island they find. The technology does not exist to bring food and other support to sufficient sized garrisons to protect each and every site. Garrisons will be found on selected islands. Abandoned islands are only resettled when there is sufficient number of colonists and sponsors available. Many islands are by-passed in favor of ones that are more recently abandoned or known to be cleared of monsters. Adventurers and other groups can easily get permission, and a grant of nobility (if appropriate) to settle an area. Almost always, the king will grant not just one island, but a cluster of islands. Protecting it's own borders by clearing the nearby islands of monsters and other threats is up to the new noble or colonizing company.
A basic strategic decision was made that having powerful nobles in scattered parts of the claimed empire was better than expanding only upon the direct edges of settled islands. The king grants a powerful noble or a company not just a single island, but a group of islands. Such scattered powerful sites would be able to defend themselves from barbarian raiders better than lots of smaller, less powerful sites that were closer together and closer to the edge of the civilized islands. This results in many islands being by-passed in resettlement. The island have been lightly explored, and previously settled, but are not important enough, economically or militarily to settle immediately. Also, such scattered powerful islands were less likely to be raided, and could even intercept raiders trying to get deeper into the empire.
Sky World uses almost exclusively renewable energy sources. Wind power is the most common source for large usage applications. Waterpower is also common, mostly on the edges of the floating islands. There is sufficient rain in most parts of the world so that excess water is always being dumped out into the atmosphere. Passive solar, including hot water, focused mirrors and concentrating lenses is well developed. Only a few societies have the technical ability to make lenses, so the trade value is quite high.
Wood, charcoal and animal dung is used for burning applications such as cooking, metal forging and similar. There is no coal, oil, or even large amounts of peat. The limited means of heating materials makes glass, metals and even fired pottery expensive. Houses are generally not heated except incidental to cooking and passive solar.
Beeswax, animal fat, vegetable oils are used for illumination. There are also varieties of magical illumination such as the AD&D style "continual light" spell. The existence of native sky based animals provides some high grade animal oil similar to whale oil. Limitations are similar to whale oil. The sky whales are relatively rare, hard to find and hard to kill. Generations of hunting have reduced the number of animals in most of the world. In general, active life stops at sunset; there is no moon of sufficient size and closeness to provide any significant illumination. Elves, Orcs and similar creatures with infravision or with light gathering eyes have an advantage and their societies will have after dark events.
Sky World consists of bodies of material floating in the sky. The lack of land creates a lack of many standard materials. Metals are uncommon, but not totally unavailable. Metal ores tend to be of low purity, requiring significant refining to achieve workable material. There is no coal, oil or other fossil fuels. Clay is common, but expensive. Clay can not be found on every floating body, but is available on most of them.
Wood, cloth and other organic materials are very common. The growing of cotton has taken a toll on the soil of lots of islands. Many cotton-farming islands have to be abandoned every few years and allow regenerating from natural overgrowth. Crop rotation is unknown.
There is a constant, albeit slow, source of material coming from the surface of the planet. Volcanic, or other activity, releases dust and debris into the atmosphere constantly. This material falls upon the islands. Over time, if an island is abandoned, such dust will cover and hide the remains of civilization and buildings. This dust, along with the penchant for underground military installations, provides lots of opportunities for dungeon adventures.
Population on a specific island is partially constrained by food production. There are plenty of islands to grow food on, but it is very difficult to distribute it between islands. Most islands are self-sufficient in basic foodstuffs. The diet might be limited, but if the population is kept in check, there is sufficient to eat.
There is no crop rotation, nor are there any advanced plows. Soils are relatively shallow in most places, and heavily farmed. The heavy European moldboard plow exists, but not the American deep plow. Humans or cattle pull plows.
Pre-Dark Age Sky World was more technologically advanced than current Sky World. A number of technologies are easily rediscovered either via research or finding examples while adventuring. Adventurers could find actual devices, or records that mention lost technologies in sufficient detail to recreate them.
Maneuver units and tactics are a lost art. The typical warfare group consists of a leader, a bunch of individually well-trained sword & shield followers and occasionally an untrained horde of spearmen or pikemen. Introducing spear or pike maneuver units is an expensive and politically difficult idea. An individual would have to pay wages and train people to fight as a unit. The existing political & military mind set would not provide money or men for such an idea. Finding a battle to prove the tactics would be tough also. Army commanders would not know what to do with such a unit beyond the normal send them into die charge. After winning a few battles though, the idea should catch on as viable, but will step on the interests of existing war bands and their politically powerful leaders and supporters.
There are no paratroopers. The idea for parachutes, especially steerable parachutes has been lost. Inventing parachutes will either require lots of basic aerodynamic research to make a working model, or finding plans or an actual old parachute. Once parachutes have been invented, getting them used as an assault method will be fast.
The concept of aircraft carriers and of specialized assault craft has been lost. Currently, all broomsticks and flying carpets are ground based or at best launched from the deck of normal ships. Having a ship that is able to quickly launch and recover lots of flying units will take money. Specialized aircraft carriers would be expensive to build, and the player character would have to do significant amounts of the design concept themselves. There would be no opposition, other than ridicule, to aircraft carriers. Specialized assault craft would be similar. The warriors understand how landing behind the lines of existing troops makes for easier looting. Designing a craft that could fight it's way in and back out would be adopted quickly, but would not be funded by anybody.