Though there was no particular base behind the languages, Ueda equates Ico's spoken language to Chinese and Yorda's language to French.
Contributed by DrakeVagabond
In response to criticisms that Ico's combat was too simplistic, Fumito Ueda stated:
"Ico's battles were just one way to create situations in which the player protects the girl. So I thought that the battles should not require the player to be very skilled."
Contributed by DrakeVagabond
There is a novelization of the game called Ico: Castle in the Mist by Japanese author Miyabe Miyuki, written out of appreciation for the game. The novel provides backstory in an alternate perspective for the characters not featured in the game, and Miyuki's personal interpretation of the story. The novel is thus considered to be non-canonical.

Miyuki thanks the games producers and creators in the closing comments for allowing her the creative freedom with the story and the use of the same cover art as the game.
Contributed by DrakeVagabond
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The Japanese and European boxart was inspired by the painting The Nostalgia of the Infinite as well as other works by painter Giorgio de Chirico. Ueda felt that the surrealism in the painting matched the allegorical world of Ico.
Contributed by DrakeVagabond
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It is possible to translate the language spoken by Yorda and The Queen. Each symbol in their subtitles represents a letter in the alphabet. The words will translate to backwards Romaji, which can then be translated to other languages.
Contributed by DrakeVagabond
In an attempt to make the game's themes and narative stand out, the team implemented a method called "Subtracting Design", in which they removed elements which interfered with the game's reality.

Ueda admits that they may have subtracted too much design.
Contributed by DrakeVagabond
Early footage shows Yorda was originally going to have horns and Ico was not. However the footage portrays Ico with horns during different segments of the clip.
Contributed by DrakeVagabond
Ueda's main inspiration for Ico was Another World, which used cinematic cutscenes and lacked any head-up display elements as to play like a movie. It also featured an emotional connection between two characters, despite the use of minimal dialog. Ueda also cited Lemmings, Flashback and the original Prince of Persia games as influences.
Contributed by DrakeVagabond
In the game's demo, the player had the option of letting go of Yorda's hand after catching her when she jumps across the bridge. This feature was removed completely in the final release.
Contributed by DrakeVagabond
After two years of development, the team ran into limitations on the PlayStation hardware and faced terminating the project altogether. However, the team decided to remain true to Ueda's vision, and shifted the project's development to the PlayStation 2.
Contributed by DrakeVagabond
Ueda wanted to make sure that a translation of Yorda speech was not included, in order to emphasize the language barrier between Ico and Yorda, and strengthen the "holding hands" concept of the game.
Contributed by DrakeVagabond
The development team was able to provide planned box art by Fumito Ueda's and additional features such as a two-player mode in time for the game's Japanese and PAL releases, but these weren't ready in time for Sony's planned North American release date.
Contributed by DrakeVagabond