The Citadel of Saladin (Qal'at Salah ad-Din) is a medieval fortified complex on a spur of the Muqattam Hills overlooking central Cairo, begun by Saladin in 1176 and expanded by every subsequent ruler for the next seven centuries. From its walls you can see the entire city — the minarets, the Nile, the pyramids on the western horizon.
The main attraction inside is the Mohammed Ali Mosque, the 19th-century Ottoman-style building with twin minarets and an alabaster-lined interior that dominates the Cairo skyline. The complex also contains the Al-Nasir Muhammad Mosque (14th century), several military museums of variable quality, and the ornate Gawhara Palace. The views from the Citadel walls are exceptional.
Getting here is easiest by taxi from downtown — the Citadel sits on a hill that makes the approach confusing on foot. Allow two to three hours for the full complex, or one hour if you're only visiting the Mohammed Ali Mosque.