This is definitely the highlight of Casablanca. The amount of finely-detailed work that has gone into this place is astonishing, and must be seen to be believed. If you're not a Muslim, the only way to get in is as part of a guided tour (these run regularly throughout the day, when the mosque isn't being used for prayers). It's well worth the Dh120 for the 45-minute tour. Take a bit of extra time either before or after your tour to admire the mosaics on the walls and patterns on the doors.