Map of Ville Vesuviane
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When visiting Naples I visited the two most famous villas: Reggia di Portici and Villa Campolieto. As far as I know, these are also the only ones among the 122 registered villas that are regularly open to visitors. The Vesuvian villas were built in the 18th century as noble private residences along the shore of the Bay of Naples, just south-east of the historical centre of Naples, stretching down to Herculaneum. The term Miglio d'Oro (Golden Mile) was coined in the 19th century referring to only about 30 of these villas built in the the vicinity of both the Reggia (the royal palace) and the archaeological site of Herculaneum and mostly taking up a boulevard of about a mile. This extravagant street reminds of Strada Nuova in Genoa, with the multiple facades in a row hiding extravagant gardens and interiors.
For touristic purposes the definition of the Vesuvian villas was laxed, but my guess is that for the sake of the proposed nomination the original delimitation was intended. The villas are inherently linked to the excavation of Roman Herculaneum by which they were inspired, both architectonically and in their wall paintings.
Both Reggia di Portici and Campolieto are in a walking distance from the archaeological WHS of Herculaneum and can be easily visited along it. Reggia di Portici is now used by the University of Naples, but it is open to the public including its beautiful gardens, which are used as a botanic garden by the university. Since it was used by as a royal residence it is not unlike other royal palaces of the time (e.g. Caserta), although its vicinity to the shore and its comparably (!) small size give it a more "rustic" impression. It is still very ornate, pompous and colourful, similar to any Italian palace of the 18th century. Campolieto shows a more Neoclassical character with its rows of Greek columns watching over the sea. Both are very picturesque. Most of the villas are in private hands and can only be seen from the street or from the regional train which now travels between the sea and the villas, spoiling the view for the poor patricians. The construction of this line (Naples-Portici) in the 19th century is also cited as the main reason for the decline of these villas.
I would strongly recommend combining a visit to these villas when in the area as they are beautiful and atmospheric. I believe Italy did right giving up on nominating them with many similar sites that are either artistically or historically more significant. This said, I am pretty sure that if the villas were located in another (European) country, a nomination would be underway and could turn successful.