The Angevin rulers decreed that executions would be held here after they beheaded the last Hohenstaufen King of Naples in 1268, sixteen year old Corradino. The next notable execution occurred after the 1647 uprising against the Spanish when they levied a high tax on fruit. The leader, Tommaso Aniello d'Amalfi, otherwise known as Masaniello, lost his life here. Thereafter, the piazza became a site used for the graves of those who died of plague. Most famously, when the Bourbons crushed the Parthenopean Republic in 1799, all its leaders were executed here, including Eleonora Pimentel Fonseca.
You can find this eerie place by walking from the train station down Corso Umberto a little ways and then taking a left down to the port. Piazza Mercato is also not far from where cruise ships come in. But hang on to your purse and bring tennis shoes just in case the neighborhood kids, whose memories are short, have set up a soccer net and ask you to play.