Castle in the Sky

Castles are the place of dreams, being in plain sight but impossible for common folk to actually reach. Airplanes and flight are also the stuff of dreams, and the collective goal of humankind for centuries is to achieve flight. Combine them together, and you'll get the Castle in the Sky. This trope is about castles that aren't just symbolically out of reach — they're also physically out of reach, suspended or cruising through midair via the power of Applied Phlebotinum. How do you get there? Probably a Cool Plane, although within the setting, using Those Magnificent Flying Machines may be more thematically appropriate.

A Castle in the Sky is Exactly What it Says on the Tin, and represents a place to strive to reach. They are often the goal of a a major quest, and often an entire subplot — if not the entire work — revolves around reaching this location. As such, it's a subtrope of Bright Castle, but, you know, flying. Such castles may be long abandoned, or only their existence passed down through generations via rumours, which will inevitably lead to an All Myths Are True realisation. People that live on one of these castles are probably prosperous and well-off, a group figuratively and literally above others.

The purpose here is to inspire a sense of wonder. Castles are magnificent, towering structures symbolically represent power. Similarly, humanity has tried for centuries to achieve flight. Combine the two together, and you create a magical location that defies our current technology level, leaving the audience in awe. It also makes a great wallpaper farm.

The phrase comes from "castles in the air" - creative ideas that are simply too difficult (if not impossible) to realize - translated from English into Japanese then back into English.

Typically where giants live. The inversion of a Ominous Floating Castle. Whereas the Ominous Floating Castle is associated with forces of evil, the Castle in the Sky is always a positive thing, and a place connotated with virtues and goodness. A Castle in the Sky may appear on a Floating Continent, or maybe it is the floating continent. For the trope namer, see Laputa: Castle in the Sky. Do not confuse with Castle in the Air.

Examples of Castle in the Sky include:

Anime and Manga

The Trope Namer is from Laputa: Castle in the Sky, which was inspired by Gulliver's Travels. The castle is much more technically advanced than the military, and possesses weapons of mass destruction. It has been long abandoned, taken over by nature, wildlife, and trees, and is fully exploited to deliver a Green Aesop. It is depicted as a serene and otherworldly location, untouched for centuries, until the military shows up and exploits it for its treasures and firepower.
Salem (a.k.a. Tiphares) in Gunnm/Battle Angel Alita is a shining, modern city floating above a filthy, crapsack Scrapyard; the place that the denizens of the Scrapyard can never quite reach. Hell, the security system even shoots down birds that dare to fly below it. Definitely not played straight: to those who know the secret of Salem, it might even be worse up there. All of the adult inhabitants have, unknowingly, had their brains replaced with a chip.
The castle in Revolutionary Girl Utena, suspended upside-down above the dueling arena, is said to be where Dios' spirit dwells. But depending on whose interpretation you accept, it's hard to tell if it's a physical castle, an illusory castle, or just a symbolic castle.
In UQ Holder, the space elevator at the capital is symbolically a Castle in the Sky.

Fairy Tales

In Jack and the Beanstalk the giant lives in one of these.

Literature

Gulliver's Travels is the inspiration of the anime Laputa: Castle in the Sky. In Gulliver's Travels, Laputa was an island that was able to levitate through magnetism, that was the home of both the royalty and the scholarly class. It served as a symbol of how these groups were detached from the real world.
In The Phantom Tollbooth, Milo's quest is to rescue the Princesses Rhyme and Reason from "The Castle in the Air".

Rhyme: ...no matter how beautiful it seems, it's still nothing but a prison.

Castle in the Air has one created by djinns, appearing like a large cloud from the distance. The book teases the trope, stating how the clouds in the sky look like a castle, before Abdullah and Sophie travel to one in the last quarter of the book. It's there in the title!

Music

Ian van Dahl's song "Castles in the Sky" (probably to no one's surprise) explores this theme.

Tabletop Games

Yu-Gi-Oh! has the Synchro Monster Cloudcastle; the older Castle of Dark Illusions is more of an Ominous Floating Castle.

Theatre

In the (English) 1980 musical adaption of Les Misérables, Cossette sings a song called "Castle on a Cloud" as a child. It represents her daydreaming of a place where she's loved and free from her abusive life with the Thernadiers.

Video Games

The castle of Queen Zeal in Chrono Trigger tops the Floating Continent of Zeal, a peaceful and enlightened land (or so it seems). A time of true enlightenment, unless you're stuck on the Earthbound Island below. Extra credit for also having an Ominous Floating Castle, the Black Omen.
One of the levels in Scribblenauts Unlimited was Storybook Keep, a castle on a cloud inhabited by fairy tale characters.