The Great Mosque
Unfortunately the Great Mosque is currently (Aug 2004) closed for refurbishment, and is not accessible at all. You can see some of the buildings from the street and from the staircase in the market building next door.
Many of the guide books infer that the old city (Guihuacheng) is the Muslim quarter. It is not; the Muslim Hui quarter is in this area, to the east of the mosque, especially the streets to the east of it. This is the area outside the old city, but just to the north of it.