Green’s Dictionary of Slang

Castle, the n.

1. see Holloway castle n.

2. (N.Z. prison, also Castle Grey Skull) Mount Eden Men's Prison, Auckland [‘Mount Eden's grey stone fac;ade, square towers and high outer wall resemble a medieval fortress’ Looser (2001); the castle inhabited by the evil ‘Skeletor’, a character from the 1980s cartoon series Masters of the Universe].

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[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 38/1: castle, the n. Mount Eden Men's Prison, Auckland [...] Castle Grey Skull n. Mount Eden Men's Prison, Auckland.