Who can resist a stone confection floating in the sky? The concept of a structure in the clouds is ancient, Jack in the Beanstalk being a more recent variation of an old theme. “Building castles in the sky” is a hoary expression for daydreaming which goes back, according to some to Don Quixote’s battle against the windmill/giant. In my own fantasy experience, it has long been a staple of my D&D campaigns. Many decades ago I even created a floor plan of an elaborate flying castle, one I’m using as a major place in my current project. My favorite Studio Ghibli film is Castle in the Sky.
Typically, flying castles are depicted much as a terrestrial castle but with many more towers and turrets. There’s nothing wrong with that, I love the imagery myself, but would a flying castle really look like that? Here I’ll poke a bit into flying castles, not to denigrate the common image but to toss out some other conceptions.
One staple of flying castles are battlements: crenellations, machiolations, turrets, barbettes, and all sort of related castle bits. Question is, do these make sense on a flying castle? These are all intended to provide a point defense against attack by ground forces, often ones at the foot of a wall. For starters, most flying castles seem to have a sheer cliff at the foot of the wall, thus providing no place for ground troops to stand and, therefore no reason for machiolations for dropping stones on to them. But step back a bit, who is going to attack a flying castle on foot anyway? You certainly can’t walk to one. To attack a flying castle, one needs to fly and flying over the wall at the castle is a bit of a trivial step having flown all the way to the castle.
That raises the next point- why have walls at all? There are similar problems with conventional towers (usually open at the top) and even crenellations: they offer protection in only one direction, outward facing, and not from above or behind.
So what might a flying castle look like? One might still have walls to contain and compartmentalize attackers, once they have landed. But since the attackers probably didn’t fly in with siege towers or even scaling ladders, the walls need not be very high or very thick. In this case, internal walls would be just as important as peripheral walls: the place might look like a honeycomb. And instead of crenellations and hordings built to face an attack from the ground and outside the wall, one could imagine crenellations that arched over the defender. Perhaps this might be a tube with arrow slits in sides and top, although that would severely limit visibility for those inside. But it could also be crenallations that had a front and top, and were set on both sides of the wall (like interdigitated but untouching fingers of two hands). Now your defender can duck behind something that provides front and top protection when he needs it but can also poke out to see around. If he needs protection from an attack from the back, he could use the crenellations on the opposite side of the wall. Of course, you could dispense with walls entirely and imagine instead a castle composed more as a series of connected blockhouses.
What to do about roofs? A flying attacker can land on a roof and either attack from that vantage point with missile weapons or break through the tiles (or what-ever serves for a covering) to make a breach. From there they can either gain entrance to the castle or just set it on fire. How to defend the roof? You could imagine spikes to keep flying creatures off the roof or some other magical defense (oil of slipperiness for instance 🙂 ). You could also imagine roof-line sally ports for defenders to shoo off attackers and multiple towers of the same height sited so that defenders in one tower can attach someone on the roof of the other.
One likely defense for such castles, although a defense with surprisingly high upkeep: netting. String nets between your walls and over the tops of your towers and it won’t be easy for a flyer to land unless he can remove the netting (dragon fire might prove handy for that.) Conventional netting, though, would break down fairly quickly high up in the air with its bright sunlight and plentiful moisture. Perhaps more exotic materials could be used or maybe the nets are only run out when attacked (although stringing out lots of netting could take a very large and skilled crew: it took many hundreds of sailors to run out the awnings in the Imperial Roman Coliseum). Or maybe the netting is actually some form of magic. In D&D terms, low-level wizards with wands of web or some other stringy magical force, not necessarily sticky.
But in the end, why have any above constructs at all? Maybe the castle is built into the floating rock itself. Now you don’t have roofs, you just have arrow slit galleries, sally ports and the like. In the end, if you have a flying mount, you can probably carry a catapult stone high above the flying fortress and just drop it on the castle. It’s pretty hard to build a roof that could resist such an attack.
Or maybe the defense isn’t the stonework. Fortifications are all passive defenses. There are active defenses. Like a modern aircraft carrier, perhaps the real protection for the castle are the aerial creatures it carries. A few fire-breathing dragons might make stonework moot. If not dragons, griffin riders or other aerial mounts could serve. Such a castle would have to have a place for their aerial defenders: hangers or caves, large sorti ports, places to land, barracks for riders and craftsmen to maintain harnesses. There might even be huge cattle pens for food for the mounts if they could not feed off the land below. Start adding in structures for aerial mounts and your flying fortress begins to look very different from a simple Middle Ages fort tossed into the sky.
Of course, there are reasons for having more conventional castles, even in the air. Whimsy or awe might cause a castle builder to dress his flying fortress with unnecessary defenses. These may serve no other function than to please the builder’s fancy or frighten a neighboring lord. Beyond that, a castle that is now in the sky may have been built on the ground and later lofted into the sky (I do that myself in my current story where the flying castles are very much conventional in design.) Finally, there aren’t that many readers who are going to quibble with your flying castle if it has stonework defenses that are unnecessary or useless for aerial combat. Sometimes a flying castle is just a cool flying castle.