In the grassy area beside The Watermill, there is a tree with a small low-poly representation of a Japanese Rhinoceros Beetle built into the model of the tree. This detail was snuck into the game by character designer and background artist Kazuhiro Numata, who recalled that he was later scolded for it by the debugging team and director Fumito Ueda.

In the Limited Edition ICO and Shadow of the Colossus Box Set for PS3, an art book titled 'Shinwa wo Tsumugu' is included. It features some interviews with members of Team ICO and among them is Junichi Hosono, who claims Ueda derived Yorda's name from the character of Hilda in the Toei film, "Horus: Prince of the Sun".

This is somewhat corroborated by the movie's presence on Ueda's shelf, which is shown in this same art book.

Hilda's character traits and arc are very different from that of Yorda, but her dynamic between the valiant Horus and the ice devil Grunwald seemed to serve as a heavy inspiration for ICO's narrative.

Though there was no particular base behind the languages, Ueda equates Ico's spoken language to Chinese and Yorda's language to French.

In response to criticisms that Ico's combat was too simplistic, Fumito Ueda stated:

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.

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.

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.

To help the game's themes, narrative, and puzzle elements stand out, the team implemented a method called "Design by Subtraction", in which they removed elements which were superfluous or interfered with the game's core concepts.

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.

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.

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.

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.

Director Fumito 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.

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.