The Great Mosque of Muhammad Ali
Although there is not much to see around, the citadel is one of the monuments to see in Cairo! It is very impressive and the view of Cairo is beautiful!
Plan a "correct" outfit ie legs and arms covered (at least 3/4) to visit the mosque (for women if you have a scarf / scarf for your hair it will be highly appreciated by locals) that tourist it remains a mosque all the same.