Mosque of Al-Hakim

Completed in 1013, the Mosque of Al-Hakim is one of Cairo's older mosques but it was rarely used as a place of worship. Instead it functioned as a Crusaders' prison, a stable, a warehouse, a boys' school and, most fittingly of all, considering the behaviour of its notorious founder, as a madhouse. The two stone minarets are the earliest surviving minarets in the city. The mosque is now used by an Ismailia group.

It's not particularly interesting architecturally, and has some of the pushiest attendants in all Cairo - don't let them bully you into giving them more baksheesh than you feel is appropriate.