The Chouara Tannery
Hidden back in the far reaches of the medina are the leather tanneries (which you can smell long before you can see). Not surprisingly, in order to get the panoramic view of the tanneries you have to go into a shop that sells leather goods--the good ones hand out mint sprigs for you to hold against your nose.
Piece of animal hide are cured first in the back in the white vats (the white is due to the inclusion of pigeon droppings in whatever they're using), and then colored in the front. Traditionally, as with most functions that require working with such ingredients, those that do this work are usually considered the lowest of the low classes.
The Medina of Fez is classified World Heritage Site #170 by UNESCO.