The leather tannery in the Fez medina features leather-making techniques unchanged since the Middle Ages. Men walk the narrow paths between huge vats of lye and colorful dyes; water wheels creak as the leather is rinsed, and buildings facing the tannery are covered with pelts hanging to dry. It smells about as bad as anything can smell. Fez is one of the best-preserved Old Cities in the Arab world. The sprawling medina is also the world’s largest car-free urban zone. The massive medina, with its high walls and winding streets, cannot be navigated without a guide or you will surely get lost.