This huge and magnificent mosque and enclosure is almost all that remains of a 9th-century city built here by an enterprising upstart named Ahmad Ibn Tulun. Ibn Tulun was the son of a Turkish slave who served in the military and amassed enough military power to establish a city on empty land near Fustat, which was then the capital of Egypt. The mosque has been subjected to a heavy-handed government restoration in the last few years, but it remains one of my favorite places to visit in the city. Lacking the spectacular interior spaces of Sultan Hassan or the view of Mohamed Ali, Ibn Tulun's innovative Iraqi-influenced architecture and massive courtyard have a power and space that the others lack.
The whole building survived relatively unscathed from the destructive process of addition and renovation that most other major mosques have suffered over the century, at least until the late 20th century, when a heavy-handed Ministry of Culture renovation "cleaned away" a good deal of material that they deemed stylistically inappropriate.
There is a 2km (about 1 1/4 mile) Koranic inscription on sycamore wood that runs around the entire mosque. Some of this wood is said to have been salvaged from Noah's ark.
Although the distinctive square minaret was closed when I last visited, you may be able to walk up. It's not too high, and the view is nice. The roof may also be open for visitors, so just ask. (It may cost you a bit more in bakshish [about LE5/90¢/45p].)