Although it is in a mosque that's closed to non-Muslims, the tomb of Fez's founder (d. 828) is considered the spiritual heart of the city and is its most-visited spot. It's also a key landmark and worth seeking out, if only to see the towering wood front doors and the profusion of columns and carpets visible from the street. The current structure dates from 1308; a 17th-century expansion added its pyramidal green roof and the medina's tallest minaret.
You can tell you're getting close when the alleys are crossed with wooden beams just above head height – allegedly a means of blocking beasts of burden and forcing everyone to approach on foot. Inside, the sultan's tomb is wrapped in an elaborately embroidered cloth that is replaced every year during the moussem.