a jail or prison (especially one that is run in a tyrannical manner)
a fortress built in Paris in the 14th century and used as a prison in the 17th and 18th centuries; it was destroyed July 14, 1789 at the start of the French Revolution
of an obscure nature; "the new insurance policy is written without cryptic or mysterious terms"; "a deep dark secret"; "the inscrutable workings of Providence"; "in its mysterious past it encompasses all the dim origins of life"- Rachel Carson; "rituals totally mystifying to visitors from other lands"