This is possibly what Twain was referring to as Pentilicon

Hymettus, also Hymettos (pronounced /haɪˈmɛtəs/ in English, Greek: Υμηττός, transliterated Ymīttós, pronounced [imiˈtos]) is a mountain range in the Athens area, East Central Greece. It is also colloquially known as Trellós or Trellóvouno (crazy mountain), a name of uncertain origin. In antiquity there was a sanctuary to Zeus Ombrios (Zeus the Rain God) on the summit with numerous offerings dating especially to the 8th-7th centuries BC; they are on the site of a military base and not currently accessibe.

Monday, August 12:
"At two in the morning we swept through the Straits of Messina, and so bright was the moonlight that Italy on the one hand and Sicily on the other seemed almost as distinctly visible as though we looked at them from the middle of a street we were traversing. The city of Messina, milk-white, and starred and spangled all over with gaslights, was a fairy spectacle. A great party of us were on deck smoking and making a noise, and waiting to see famous Scylla and Charybdis."

"No--Pompeii is no longer a buried city. It is a city of hundreds and hundreds of roofless houses, and a tangled maze of streets where one could easily get lost, without a guide, and have to sleep in some ghostly palace that had known no living tenant since that awful November night of eighteen centuries ago." (Page 329)

Twain writes of Going to a town called Annunciation prior to his climbing Mt. Vesuvius. Torre Annunziata is the only locale I can find nearby Naples that fit his description.

Torre Annunziata is a city and comune in the province of Naples, region of Campania in Italy. It is located on the Gulf of Naples at the foot of Mt. Vesuvius.
https://en.wikipedia.org/wiki/Torre_Annunziata

Sam and companions climbed Mt. Vesuvius August 9th or 10th. while the Quaker City remained in Quarantine at Naples. Mark Twain Project: Quaker City Itinerary

The Vesuvius Observatory is the surveillance centre for monitoring the three volcanoes which threaten the Campanian region of Italy: Mount Vesuvius, Campi Flegrei and Ischia. Founded in 1841 on the slopes of Mount Vesuvius (the operative center is now based in Naples) by Ferdinando II di Borbone, king of "Due Sicile", it is the oldest volcanology institute in the world.
https://en.wikipedia.org/wiki/Vesuvius_Observatory