Miyazaki first visited Wales on a research trip in 1985, when Castle in the Sky was in early stages of production. He decided to take inspiration from the architecture of the region, and as a result, some of the structures seen in the film resemble Welsh mining towns. Miyazaki also witnessed the coal miners' strike in protest of mine closures in Britain. Their ultimate failure to preserve the industry left a lasting impact on Miyazaki, who viewed the event as an attack by those in power on the miners' way of life and the hard-working spirit of the people. His experiences reflect in several supporting characters in the film, who despite laboring through poverty in the mines, enthusiastically protect the protagonists from multiple aggressors. Susan J. Napier argues that this depiction reveals Miyazaki's yearning for a simpler way of life, and a desire to create a story based on optimism. Animation scholar Helen McCarthy writes "It seems that Castle in the Sky also contains echoes of the struggle of the Welsh people for nationhood and freedom." Miyazaki would visit Wales once more in 1986, ahead of the release of the film. In 2005, he told The Guardian "I admired those men, I admired the way they battled to save their way of life, just as the coal miners in Japan did. Many people of my generation see the miners as a symbol; a dying breed of fighting men. Now they are gone."

Certain special effects from the film use a combination of cel and film techniques. Takahata, who produced the film, insisted that the highest quality be maintained in spite of the production expense. Napier argues that the production of Castle in the Sky "established a new industry standard". Miyazaki stated in the original project proposal that "[Castle in the Sky] is a project to bring animation back to its roots."

Many of Miyazaki's old colleagues as well as much of the production crew of Nausicaä of the Valley of the Wind were employed once again to work on Castle in the Sky at Studio Ghibli's inception. The film had a reported production budget of ¥500 million, equivalent to US$8 million in 2023. Several animation studios such as Doga Kobo and Oh! Production provided support for the in-between animation.

Castle in the Sky contains a strong theme of environmentalism, questioning humanity's relationship with nature and the role of technology. McCarthy interprets the giant tree of Laputa as a "metaphor for the reviving and life-giving power of nature." However, in contrast with the more optimistic conclusions of Miyazaki's previous works, Napier notes that the film ends with an "unsettling view" of the castle flying away, suggesting that humanity may not deserve to exist in the natural world. Literary scholar Anthony Lioi interprets Laputa as an ecological utopia that demonstrates the peace that can be established between nature and advanced technology, but also serves as a criticism of modernity when "[the] peace is shattered by human violence." Lioi notes that this outlook differs from dominant Western ideas, eschewing the extremes of capitalism and industrialism, as well as radical environmentalism and conservationism.

Critics note the philosophical ambiguity of the castle; while Laputa initially appears to be an ideal union of nature and technology, it is later revealed to have a much harsher and more oppressive underside; Napier writes that Laputa is "deeply paradoxical". Laputa itself takes direct inspiration from the island of the same name from Gulliver's Travels (1726), and film scholar Cristina Cardia claims that, like its namesake, the island is introduced with benign intentions but is ultimately "exploited for perverse ends, in this case war." Lioi argues that Laputa is used as a means to comment on the ethics of contemporary culture, based on Ildney Cavalcanti's observation that such a utopia also "must contain an overtly dystopian element, such that the implicit critique in utopian discourse becomes explicit." However, he interprets the ultimate destruction of the castle's weapons as a demonstration that "violence is not the heart of the city", and that the dystopic elements of modernity can be healed.

The film also presents an ambiguous view on the usage of technology. The robots from Laputa provide an example of this view, as they are introduced in the film as a violent force capable of extreme destruction. However, when the protagonists next meet a robot, it is entirely peaceful, tending to the gardens and fauna on Laputa. Lioi argues that the robots, as a representation of Laputan technology, are caretakers by default and only become destructive in response to human brutality. McCarthy argues that "this is not a comment on technology but on man's inability to use it wisely." Odell and Le Blanc conclude that "technology ... is not necessarily a bad thing, but we must consider how it's used and to what extent." The duality of nature and technology is further explored in Miyazaki's later film Princess Mononoke (1997).

Like many other films by Miyazaki, Castle in the Sky features young children as protagonists. Miyazaki values the portrayal of children as good-hearted, confident in their own agency, and resilient and upbeat in response to adversity. He criticized reviewers of his television series Future Boy Conan (1978) who described the titular character as "too much of a goody-two-shoes", admitting he was tempted to retort "So you want to see 'bad characters', you fool?" Film critics Colin Odell and Michelle Le Blanc argue that creating a film with younger protagonists generates perspectives that an adult would not perceive, saying "the children in Ghibli's films are a liberating force that allows anything to be possible."

The lack of parental oversight of the protagonists is an element Miyazaki feels to be important in promoting children's independence. The protagonists of his films are, like Sheeta and Pazu, often orphaned, or in some way parted from their parents. Miyazaki believes that "one of the essential elements of most classical children's literature is that the children in the stories actually fend for themselves." The presence of parents, in his opinion, would stifle the children's autonomy. The limitations that children have in their abilities are also explored in the film; for example, Pazu comes close to forsaking Sheeta and his quest for Laputa. Additionally, unlike Miyazaki's previous works, the protagonists do not succeed at convincing the antagonists of their wrongdoing, which offers a more pessimistic view on children's ability to educate others.

Napier proposes that Miyazaki's insistence on showing the freedom of children in Castle in the Sky can be credited to the influence of Panda and the Magic Serpent (1958). Miyazaki first watched the film at age 17, and it moved him to pursue a career in animation. At a lecture given in 1982 at Waseda University, he said "When I saw Panda and the Magic Serpent, it was as if the scales fell from my eyes; I realized that I should depict the honesty and goodness of children in my work." He considers this a focal point in his endeavors. The theme of innocence is explored further in Miyazaki's succeeding film My Neighbor Totoro (1988).

Miyazaki's affinity for flight is repeatedly displayed in Castle in the Sky, a motif that continues throughout the feature films of his career. A variety of fictional flying machines appear across the film, including the island of Laputa, the airships, and the pirates' ornithopters; Sheeta's crystal also allows her to float through the air. However, many of the other flying machines in the film are retrofuturistic, influenced by nineteenth-century stylistic approaches. Additionally, Miyazaki was inspired by the literature of Jules Verne and Robert Louis Stevenson when considering the style of the film.

Another stylistic trait that Miyazaki drew from nineteenth-century influences is the depiction of machines that "still possess the inherent warmth of handcrafted things." Literary scholars Jeff VanderMeer and S. J. Chambers argue that Pazu's enthusiasm to build and work with flying machines gives the film's airships "a realistic physicality." Commenting on the mecha anime popular at the time, Miyazaki expressed his hatred for shows that glorified machines without portraying the characters struggling to build or maintain them. Animation scholar Thomas Lamarre argues that the film provides an "alternative to our received technologies" and thus a critique of more contemporary technologies and society's perception of them.

Boyes felt that many of these elements subsequently influenced the steampunk genre. Napier writes that Verne's impact on the film's style was instrumental in evoking imagined nostalgia for a time when "machines were still fun", in Miyazaki's words. While the other machines are presented as joyful, Laputa's underside is used exclusively as an instrument of destruction. Miyazaki stated that, as a child, he was attracted to the design and power of military planes, a view that has since been replaced with revulsion for the indiscriminate acts of violence that the machines have been used for. Miyazaki further explores the beauty of flying machines as well as their innate destructive potential in his later film The Wind Rises (2013).

The film was released in Japan on August 2, 1986, by the Toei Company. At the Japanese box office, the film sold about 775,000 tickets, somewhat lower than the performance of Nausicaä of the Valley of the Wind. Miyazaki and Suzuki expressed their disappointment with the film's box office figures. The film was promoted with a tie-in fruit soda brand which animation scholar Rayna Denison described as an "economic failure".

The film later earned a significant additional amount through rereleases; as of 2021, it has grossed approximately US$157 million in box office, home video, and soundtrack sales. In the United Kingdom, it was 2019's eighth-best-selling foreign language film on home video, below five other Studio Ghibli films. The film has sold approximately 1.1 million tickets in Europe as of 2023. Multiple international theatrical rescreenings between 2003 and 2023 have earned the film approximately US$6.2 million.

The first English dub of Castle in the Sky was produced by an unknown party commissioned by Tokuma Shoten in 1988 for viewing on international flights on Japan Airlines; this dub was licensed between 1989 and 1991 by the then-new Streamline Pictures for distribution in North American markets. An edited version of this dub briefly aired on UK television. In addition to distribution rights, Streamline would go on to dub two other Studio Ghibli films in-house: My Neighbor Totoro and Kiki's Delivery Service (1989).

The English dub produced by Disney was recorded in 1998 and planned for release on video in 1999, but the release was postponed after Princess Mononoke did not perform well in North American theaters. The film premiered at the New York International Children's Film Festival on February 2, 2000. It was released on home video in North America on April 15, 2003, alongside a rerelease of Kiki's Delivery Service and Spirited Away (2001). Due to the possible confusion of the title with the Spanish phrase la puta – literally 'the whore' – the film was released as simply Castle in the Sky in North America. The film was released by Buena Vista on Blu-ray in North America on May 22, 2012. Shout! Factory and GKIDS re-issued the film on Blu-ray and DVD on October 31, 2017. Both the original Japanese version and the 2003 English dub were made available for streaming when the rights to Studio Ghibli's filmography were acquired by Netflix in 2020.

As with Nausicaä of the Valley of the Wind, Joe Hisaishi composed the soundtrack of Castle in the Sky. Miyazaki and Hisaishi went on to become close collaborators, and Hisaishi has since provided the music for all of Miyazaki's feature films. Three months before the film's theatrical release, the image album – a collection of demos and musical sketches that serve as a precursor to the finished score – was published by Tokuma on compact disc. A third version of the soundtrack, rearranged for full symphony orchestra and recorded by the Tokyo City Philharmonic, released in 1987 on compact disc.

For the English dub produced by Disney in 1998, Hisaishi was called upon to rewrite the soundtrack to be more suitable for audiences in America. The new soundtrack was recorded by the Seattle Symphony and featured in the 2003 English dub released by Buena Vista. Hisaishi was advised by Disney staff that non-Japanese audiences prefer comparatively more music in films. As a result, the American soundtrack is much longer, while the original Japanese version featured just an hour of music for a film exceeding two hours in length. Though Hisaishi felt that American film scores used an overly simplistic compositional approach, he commented "But when I redid the music of Laputa this way, I learned a lot."

The credits sequence of the film features an original vocal song titled "Carrying You" performed by Azumi Inoue, with music by Hisaishi and lyrics by Miyazaki. The song was released in 1988 as a compact disc single, featuring an additional chorus version performed by the Suginami Children's Choir.

Castle in the Sky has been generally acclaimed by film critics in the years since its release. In 2001, Animage ranked Castle in the Sky 44th in their list of top 100 anime. Animation critic and writer Raz Greenberg calls Castle in the Sky "one of the greatest adventure films ever made", and critic Manabu Murase names it "quite possibly the most entertaining anime that Miyazaki ever made". On the review aggregator website Rotten Tomatoes, the film holds an approval rate of 96% from 28 critics, with an average rating of 7.6 out of 10. The site's critic consensus reads, "With a storytelling palette as rich and brilliant as its animation, Castle in the Sky thrillingly encapsulates Studio Ghibli's unique strengths." At Metacritic, the film has a weighted average score of 78 out of 100 based on seven critics, indicating "generally favorable reviews".

While multiple reviewers felt that the film's two-hour runtime would turn audiences away, The New York Times's Caryn James commenting that it is "liable to strain patiences of adults and the attention spans of children", others argued that the film had the appeal to keep audiences entertained. IGN's Jeremy Conrad felt the characters are "so likable that you never get bored, you always want to see what adventure is next for them". Reviewers were split over the 1988 English dub, with the Dayton Daily News's Terry Lawson calling it "the film's weakest element", while The Cincinnati Post's David Lyman felt the dubbing into English had been done "superbly". The 2003 dub similarly received mixed reviews, with The A.V. Club's Tasha Robinson calling Disney's recordings "almost comically bland", and Conrad expressing his appreciation for Anna Paquin as Sheeta and Mark Hamill's performance as Muska. Many critics also praised the animation, the Asheville Times's Tom Sabulis considering it "state-of-the-art" and The Philadelphia Inquirer's Steven Rea naming it "masterful". However, some felt the motions lacked fluidity, with Lyman describing it as "stiff-limbed".

Most reviewers highlighted the imaginative capacity that Miyazaki displays in the film. Slant's Chuck Bowen noted the subtle details included in the film, which he felt lends it "texture and originality". A review in the Weekly Asahi highlighted the film's dynamism, favorably comparing its flying sequences with Peter Pan (1953). Several reviewers praised the use of color, which made the film "a joy to watch" according to James. A reviewer for City Road noted that the film could present themes that were critical of modern society while still maintaining a "warm and caring" view of humanity. Several reviewers noted the film's strong ecological theme, with The Washington Post's Richard Harrington appreciating the "moral duality" of Laputa's technology, and the Asahi Journal's Yomota Inuhiko praising the depiction of Laputa as a utopia which gradually developed dystopic elements. Critics and scholars also noted the film's connections to Miyazaki's previous works; Greenberg felt that the film is "deeply rooted within Miyazaki's filmography of the two decades that preceded it", and Denison called it a "compendium of Miyazaki's previous interests as an animator".

In a 2006 poll by Japan's Agency for Cultural Affairs conducted at the 2006 Japan Media Arts Festival, the film was rated the third-best animation of all time, after Nausicaä of the Valley of the Wind and Neon Genesis Evangelion (1995). In a 2008 audience poll conducted by Oricon in Japan, Castle in the Sky was voted the best animation of all time.

Castle in the Sky is considered by some scholars and writers to be an important work in the modern steampunk and dieselpunk styles. Along with Nausicaä of the Valley of the Wind, Philip Boyes of Eurogamer considers Castle in the Sky a major contributor to the genres' popularity in Japan, introducing audiences to stylistic features such as airships which were otherwise mostly prevalent in Europe. According to McCarthy, "its mix of epic action-adventure and techno-ecological theme has since earned [Castle in the Sky] cult status."

Several notable artists in the anime and manga industries have cited Castle in the Sky as a major influence on their works. Nadia: The Secret of Blue Water (1990), by Neon Genesis Evangelion director Hideaki Anno, is noted for its similarities in premise with Castle in the Sky. Anno had previously worked with Miyazaki on the production of Nausicaä of the Valley of the Wind, and has stated that Nadia was based in part on one of Miyazaki's concepts. D.Gray-man (2004) author Katsura Hoshino was moved to pursue a career in animation after watching the film, ultimately becoming a manga artist. Ghost in the Shell (1995) director Mamoru Oshii and Your Name (2016) director Makoto Shinkai named Castle in the Sky among their favorite animations. Additionally, VanderMeer and Chambers argue that Castle in the Sky forms the stylistic foundation for several of Miyazaki's later films, including Porco Rosso (1992) and Howl's Moving Castle (2004).

John Lasseter, former chief creative officer at Pixar and Disney Animation, often cited Miyazaki and his works to be his "greatest inspiration". When asked about some of his favorite films, Lasseter expressed his admiration for Castle in the Sky. Lasseter has worked with Miyazaki on the English dubs of several of his films, and notes this as an influence on his work with his colleagues. At a speech delivered at the 2014 Tokyo International Film Festival, Lasseter said "Whenever we get stuck at Pixar or Disney, I put on a Miyazaki film sequence or two, just to get us inspired again." Napier argues that the protagonist of Pixar's WALL-E (2008), a robot left to care for a world abandoned by humans, "may have its roots in Laputa's nurturing robot."

Castle in the Sky has influenced numerous video games, particularly in Japan, with its success leading to a wave of steampunk video games. Game designer Hironobu Sakaguchi cited Castle in the Sky as an inspiration behind his Final Fantasy video game series, particularly citing it as an influence on the series' airships. According to Boyes, Castle in the Sky also influenced the airships in the Mario and Civilization franchises. The Iron Golem from Minecraft (2011) takes inspiration from the robots in the film. Several games from The Legend of Zelda series are noted to have been influenced by Castle in the Sky, particularly Tears of the Kingdom (2023), which features a flying castle and several thematic parallels with the film.

On December 9, 2011, during an airing of Castle in the Sky on Japanese television, fans posting to Twitter set a new record for the platform by causing a peak of 25,088 tweets per second. The record was later surpassed during another airing on August 2, 2013, with a figure of 143,199 per second.

• Official website
• Castle in the Sky at IMDb
• Castle in the Sky (film) at Anime News Network's encyclopedia
• Castle in the Sky at Rotten Tomatoes
• Castle in the Sky at Metacritic
• Laputa: Castle in the Sky at Nausicaa.net
• 天空の城ラピュタ (Tenkū no Shiro Rapyuta) at the Japanese Movie Database (Japanese)