With a history spanning thousands of years (depending how you choose to look at it), Cairo has loads of palaces and other historic buildings, though no castles as such. For the purposes of this list, I've chosen to ignore the Pharaonic monuments, and concentrate on the most important mosques, churches, palaces, and other historic buildings.
Arguably the most important historic building in Cairo is the Mosque of Amr Ibn al-Aas, since this was the first ever built in Africa, and marked the founding of Fustat, which grew into Cairo proper under the Fatimids. Al-Azhar Mosque is also of great significance, being the de facto spiritual seat of Sunni Islam, and home to one of the oldest universities in the world. Other important mosques include Ibn Tulun - the oldest in Cairo that still survives largely in its original form, and boasting a unique spiral minaret - and al-Hussein Mosque - considered by many to be the holiest in Egypt.
Cairo churches such as the Church of Abu Serga are of great historic significance, since this is where the Holy Family are said to have sheltered when they fled to Egypt, and the Hanging Church is a beautiful historic building with a suspended nave.
Beit al-Souhaymi is a well-restored example of an Ottoman house, arguably the finest in Cairo, and the Gayer-Anderson Museum - adjoining Ibn Tulun - is a house that used to belong to a British army major, and is crammed with his eclectic and obsessive collections of art from all over the Middle and Near East.
Finally, including the Citadel is perhaps a bit of a cheat, since it's not just one building. But it is the closest thing Cairo has to a castle, and contains three significant mosques from different periods of Cairo's history. Well worth checking out!
Sidi Hasan al-Anwar Street
Opposite El-Hussein Square
Egypt's Highest Islamic Authority... read more
Darb al-Asfar Street
Around Bab el Futuh
Beautifully restored old Cairene house... read more
Mar Girgis Street
Lavish, suspended church; conducts mass in Coptic language... read more
Off 'Abd al-Magid al-Labban (Al-Salbiyya) Street
Ibn Tulun Street
Stunning architecture and unique minaret... read more
Near Mari Girgis Street
Supposed Resting Place of the Holy Family... read more
Egypt's most sacred mosque, housing grisly body part... read more
Southern tip of Roda Island
4 Maydan Ibn Tulun
Ibn Tulun Street
Eclectic museum in beautiful home, used in James Bond film... read more
Salah Salem Highway
Imposing fortress with fantastic views, mosques and museums... read more