This game was being developed by Production Studio 4 alongside Resident Evil and was announced in 2002 as part of the CAPCOM Five agreement. In the West it is otherwise known as the "Fog" or "Mist" version, and is recognized as such by developers.
This version's premise was to feature Leon S. Kennedy infiltrating a European castle belonging to Oswell E. Spencer as part of a larger US-STRATCOM assault. Coinciding with this assault is an attack by H.C.F. on the estate, who unleash new bacteria-sized virus into the fight. Both forces are destroyed during the attack. While Leon escapes death, he is infected by the virus.
Over time Leon was to discover the origins of the Progenitor Virus. In the backstory, it was found in the corpse of an ancient human by archaeologists exploring the castle, and gave Spencer the ambition to begin his virus research.
A sideplot is said to have involved a female partner character waking up from her sleep in the castle and exploring it with her B.O.W. dog.
One of the most notable areas shown in this version was a flying airship.
In keeping with Hideki Kamiya's suggestions for the "Stylish" BIOHAZARD 4, "Castle" uses 3D settings rather than the pre-rendered backgrounds used in older games.
Castle was first unveiled at the Tokyo Game Show's Spring 2002 Expo, with Hiroshi Shibata attached as the Director. The story was written by Flagship writer Noboru Sugimura, and Resident Evil 3: Nemesis writer, Yasuhisa Kawamura.
"Castle" ran into a series of development problems. For example, the Black Fog enemy proved too difficult to animate on GameCube hardware. Capcom ran into the same problem with the Uroboros enemies in Resident Evil 5, but solved it by developing a new animation technique.
Eventually, the scenario was scrapped to allow development to start over. Noboru Sugimura handed his scenario draft over to Capcom's Production Studio 3, which shifted focus to the unnamed girl and her dog to develop a new game. The resulting game, Haunting Ground, was released in 2005 for the PlayStation 2.
- Leon's left arm was to be infected at some point. To prevent the infection from spreading, Leon would wrap a belt around his arm.
- Wesker and the H.C.F. were to play a large role in the story as it was a direct sequel to Resident Evil CODE:Veronica, with Veronica's writer Noboru Sugimura also attached to the project.
- Capcom's Fantastic Five. IGN (November 13, 2002). Retrieved on July 17, 2010.
- "Im Gespräch mit Shinji Mikami" (in German). Play (CyPress GmbH) (05/2001): 27. May 2001. "Mikami: Ja, Flagship hat die Story dazu geschrieben. Mehr über Teil 4 werde ich aber nicht verraten (lacht). / Yes, Flagship wrote the story for it. But I won't reveal anything more about part 4 (laughs).".