The Fortress of Salah Al-Din is a major must see in Cairo, it was actually one of the first things I visited and it's definitely worth a visit at both day and night because it looks good both lit up artificially and naturally. Considering the work that has gone in to this masterpiece and the buildings the Citadel complex holds, it's difficult for it to look anything but magnificent.
Basically the Fortress was built by Salah-Al-Din in AD1183 and the idea of the citadel was to protect both Cairo and the early capital of Futsat having just fought off the Fatimid Caliphate. Over the next 200 or so years, work continued and this remained the head of the Egyptian government until the late 1800s when it moved to a different palace.
Not only do you get a stunning view from the outside walls of the citadel and a great view of the city from the inside but this houses some spectacular and informative buildings. Within the complex, you can find the Al Ghawara Palace, the National Military Museum and a Police Museum, perhaps the most awesome building of all though is the impressive Mosque of Mohamed Ali Pasha built over two decades starting in the late 1820s.
One of the things I recommend doing is going there on a Monday, Wednesday or Saturday at around 7pm, this is best in winter obviously because by then it's already dark and you can kill two pigeons with one stone (you get to see the building at dark), they have a free sufi dance performance by the whirling dervishes which lasts almost an hour.
It's truly amazing and like nothing I'd seen before, what's more is it's the real mc coy and not the commercial type you may find in Turkey, it's also for free as I mentioned above. The ceremony is an important part of the Sufi Muslim culture and they spin round in circles to hypnotic drumming in colourful clothes in a transient state. If you get there early, you should be able to get a front row seat - it's a crazy colourful display and looks inhuman!