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Enter these at the main menu choosing 'Options' and then choosing 'Codes' on the option menu.

Cheat	Effect
SACRED FEMININE	Extended Health
VITRUVIAN MAN	Invincibility
PHILLIPS EXETER	One-hit kills
ROYAL HOLLOWAY	One-hit weapon kills
CLOS LUCE 1519	Unlock all levels
ET IN ARCADIA EGO	Unlock all secrets
APOCRYPHA	Unlock all Visual Database items