Naples’s Quartieri Spagnoli, the “Spanish Quarters,” are a part of the city with a long and tumultuous history. Founded in 1500, the Quartieri Spagnoli were created by Don Pedro De Toledo to accommodate the Spanish soldiers who were residing or passing through Naples. With the arrival of the soldiers, the network of narrow streets became a hotbed for illegal economic activities, from cigarette smuggling to drug dealing to prostitution, earning the district a bad reputation that stuck for centuries – even Neapolitans from other neighborhoods were afraid of entering the Quartieri Spagnoli. [Source]
For me, wandering the Spanish Quarter was one of the best days i spent in Napoli last year.