Zelatar
Zelatar is Graz'zt's capital city, crossing over the three (3) layers of Azzagrat, #45-7 of the Abyss. Zelatar is considered one of the safest (although still dangerous to the unprepared) and most civilized spots in all the Abyss (although that's admittedly not saying much).
This makes it incredibly (for) popular for those wanting to deal in all the illegal goodies one can only hope to find in the Abyss - which is how its Demon Prince likes it. The only things forbidden in this city are restraint and kindness.