The acrid smell assaulting your nose announces your arrival in Marrakesh's tannery area. You'll find tanneries scattered on either side of Rue de Bab Debbagh – generally with touts stationed at the gates, offering tours for a tip. The best time to come is in the morning when you'll usually be able to see tanners at work, transforming leather hides into a rainbow of hues. It’s hard, dirty work and dangerous too, now that natural dyes have been eschewed for chemical colours.
In exchange for a tip, you'll usually also be offered to see a view of the tanneries from above, from one of the houses near the Bab Debbagh gate. The views are definitely worth it, but be aware that many of the 'houses' are actually leatherware shops and touts can be pushy. Don't feel pressured into having to buy something if you don't want to.