Habs Qara (Prison of the Christian Slaves): A huge underground prison where Moulay Ismail allegedly kept Christian prisoners.
Built as a prison by the Portuguese architect Cara, himself a prisoner, who earned his freedom by constructing these immense subterranean slave quarters.
60,000 slaves (of which 40,000 were reportedly Christian prisoners of war) were shackled to the wall, forced to sleep standing up, and ordered to work on the sultan's laborious building projects.