Leather tanning
Historical sources show that the tanneries were a major industry even in the city's early history and tied to a large part of its economy. 

The products of the city's tanneries were also prestigious enough that they were reportedly exported all the way to Baghdad. Al-Jazna'i claims that the Almohads (late 12th to early 13th century) counted a total of 86 tanning workshops in the city, while a later source claims that there were around a hundred in the Marinid period (late 13th to 15th centuries). 

The tanneries continued to be expanded or modified on several occasions even into modern times. 

Source: Wikipedia 

The Médina of Fes was listed as a World Heritage Site in 1981.
morocco fes fez medina leather leathertanning tanning tannery tannerie maroc worldheritagesite worldheritage