- "This place seems to be inside the game, but at the same time, seperate. It's outside some sort of boundary!"
- — Bear —
Castle is the fourteenth episode of .hack//SIGN.
The episode begins with Sora following Crim and BT into the red Chaos Gate just as the Twilight Eye closes and the area normalizes. Crim and BT decide to head to the Gott Statue, which is the most likely place the writer of the Key of the Twilight post is. They both soon become aware of Sora's presence and increase their pace.
It soon becomes a race to the Gott Statue between Sora, Crim and BT, and Bear's party. Tsukasa is focused on finding the Key despite Bear and Mimiru acknowledging the danger involved. Crim stays behind to stall Sora while BT moves ahead. As BT looks back, she smiles knowing her plan has gone perfectly. As Crim and Sora fight, they both acknowledge that they were manipulated by BT, who reaches the sealed room first. BT leaves the room just as Bear arrives. She acts strangely, as if scared of something she learned in the room. She claims that the Key was not inside and suggests that Bear's party enter to understand.
Inside the room is an AI that repeats a vague, regretful message. It speaks of the "proof of love made tangible" and his daughter who holds his last hope. Tsukasa understands that he is referring to the sleeping girl he knows and states that he may have found the Key. The area soon begins to collapse, and all players within must run. Bear soon falls due to a collapsing bridge, but Mimiru and Tsukasa have no time to act, as Macha appears in front of them and summons a Guardian. The Guardian attacks Mimiru, but Tsukasa pushes her out of the way, causing the attack to strike and kill Tsukasa instead.
- The upside down castle makes another appearance as a bonus dungeon in Mutation, using the same keywords, Θ Bitter Fantasy Mirror World.