The manner in which The Citadel dominates the skyline of Cairo is largely due to the efforts of an Iraqi Kurd named Salah el Din. Salah entered Egypt around 1163 with an army sent by Syrian ruler Nur el Din. By 1171, he had made himself ruler of the country and set about modernizing the defenses of Cairo. The cornerstone of his plans was the magnificent, and virtually impregnable, walled compound that he built on the eastern edge of the city. It was completed in 1183, when Salah left Egypt to fight the crusaders. By the time he had expelled them from Jerusalem in 1187, Salah was well known, and rightfully feared, in Europe as Saladin.
The Citadel is perhaps most famously associated with another great leader, Mohamed Ali, who ruled Egypt in the early 19th century. You can still see the alleyway below The Citadel where his soldiers slaughtered almost the entire nobility of Egypt one night in an after-dinner ambush.
The compound is quite large and contains a number of museums and mosques. It was still a military facility until fairly recently, and though the museums aren't to Western standards, the aura and atmosphere of the place, not to mention the view over the city, make this an afternoon or morning well spent.
The Mohamed Ali Mosque is my favorite mosque in Cairo because of its huge, typically Ottoman dome, which will be familiar to anyone who has visited Istanbul. It's usually not very crowded, and because it sits on the edge of the escarpment over Cairo, it has an unusual amount of light and fresh air. The view from the courtyard over Cairo is the best in the city, and if you time your visit properly (I recommend sunset), you can hear the call to prayer rising up from the various mosques around town.
The Coach Museum houses a small but well-preserved collection of ornate coaches used by the royal family and ranking government figures from before 1952. At Gawhara Palace, get a taste of how the other half lived by touring one of Mohamed Ali's palaces. It is, I have to say, pretty musty, and the re-created throne-room scene is mediocre at best. I wouldn't travel up to The Citadel just for the Gawhara, but if you're already here, it doesn't hurt to have a look.
The Military Museum is a musty old place full of moth-eaten dioramas and misspelled placards, but the collection of colonial uniforms and weaponry from the 1970s will interest war buffs. I have never fully understood the positive reviews the Police Museum gets. There are some interesting displays, but you get the feeling that all the best stuff ended up in somebody's house. The best that can be said for it, in my opinion, is that the damp smell and the peeling plaster will give you a good idea of what the inside of a real police station in Egypt is like.