Rokkenjima (六軒島, lit. "Six House Island") is a fictional island that serves as the main setting for the story. It is privately owned and inhabited by the Ushiromiya main family and staffed by several servants.
Rokkenjima is a small island located off the coast of Tokyo in the Izu archipelago. It measures about 10km around. The majority of the island is covered with a dense forest, with the exception of areas of land developed by Kinzo for the mansion. To get to there, the Ushiromiya family takes a plane to Niijima airport followed by a boat ride to the island.
In the distant past, the island was referred to by the name Azukishima (小豆島, lit. "Azuki Bean Island"). A story whispered among fishermen holds that this name is a mispronunciation of Akujikishima (悪食島, lit. "Evil Appetite Island"). It was whispered that the island was a gathering place for evil spirits, who were then later sealed by a shrine built by a travelling shugenja.
In Requiem of the Golden Witch, it is revealed that the island was used in World War II as a military base for the Japanese army. It was meant to serve as a launching point for the navy's Kaiten torpedoes, but was never supplied with them due to the nation's poor financial state, and eventually forgotten by the military command. In 1944, Kinzo was stationed on the island as a soldier when the island came into contact with a submarine containing soldiers from the Italian Social Republic. It is during this incident where he met Beatrice Castiglioni and acquired the legendary ten tons of gold. A gunfight eventually broke out between the Japanese and Italians over the possession of the gold, leaving Kinzo and Beatrice as the only survivors.
The island was then purchased by Kinzo from the Tokyo metropolis utilizing his connections with the GHQ. It was acquired under the pretext of establishing a military base, but then converted into Kinzo's own personal island. The island was then developed, with the mansion constructed in 1952. Kinzo's wife and children moved onto the island with him, and the residence of the island eventually shifted to Krauss's family.
After the events on October 5, 1986, the island was repossessed by the Tokyo Metropolis and sealed off from the public.