Al Qal’a of Beni Hammad

In a mountainous site of extraordinary beauty, the ruins of the first capital of the Hammadid emirs, founded in 1007 and demolished in 1152, provide an authentic picture of a fortified Muslim city. The mosque, whose prayer room has 13 aisles with eight bays, is one of the largest in Algeria. [source]

I haven’t been here (yet).