Northeast of the Kairouine Mosque and Seffarine Square is Fes's most visited -- and aromatic -- sight. A picture straight out of medieval times, the Chouwara Tannery is the largest and busiest of the four traditional tanneries still operating in the medina. Sheep, goat, and cow skins are cured, stretched, scraped, and dyed in numerous honeycombed earthen pits in a process that is still as manual as when it began in the 13th century upon the decline of Cordoba. The skins are cured by a concoction that includes cow urine and pigeon guano and are then laid atop the rooftops to dry. The final dying process involves the leather being mixed and soaked by hand in the vats before being trimmed for the medina's leatherworkers. The vegetable dyes -- poppy (red), turmeric (yellow), mint (green), indigo (blue) -- have been replaced by chemicals that, along with the odd rinsing machine, are the only modern concessions to have infiltrated the process. It's both a confronting and fascinating view at the same time, and strongly challenges your sense of voyeuristic tourism. Perhaps the knowledge that the tanneries are still one of the most important sources of income and trade for the city may temper any misgivings. The Chouwara tanneries are best viewed in the morning, from the roof-terrace leatherware shops along Derb Chouwara, where a well-placed sprig of mint will help those who find the smell perhaps more confronting than the sight.