The Dardanelles (/dɑrdəˈnɛlz/; Turkish: Çanakkale Boğazı, Greek: Δαρδανέλλια, Dardanellia), formerly known as Hellespont (/ˈhɛlɨspɒnt/; Greek: Ἑλλήσποντος, Hellespontos, literally "Sea of Helle"), is a narrow strait in northwestern Turkey connecting the Aegean Sea to the Sea of Marmara. It is one of the Turkish Straits, along with its counterpart, the Bosphorus. It is located at approximately 40°13′N 26°26′E.

The Temple of Hephaestus, also known as the Hephaisteion or earlier as the Theseion, is a well-preserved Greek temple; it remains standing largely as built. It is a Doric peripteral temple, and is located at the north-west side of the Agora of Athens, on top of the Agoraios Kolonos hill. From the 7th century until 1834, it served as the Greek Orthodox church of St. George Akamates.
https://en.wikipedia.org/wiki/Temple_of_Hephaestus

The Areopagus or Areios Pagos (Ancient Greek: Ἄρειος Πάγος) is the "Rock of Ares", north-west of the Acropolis, which in classical times functioned as the high Court of Appeal for criminal and civil cases in Athens. Ares was supposed to have been tried here by the gods for the murder of Poseidon's son Alirrothios (a typical example of an aetiological myth).

A Propylaea, Propylea or Propylaia (/ˌprɒpɪˈliːə/; Greek: Προπύλαια) is any monumental gateway based on the original Propylaea that serves as the entrance to the Acropolis in Athens. The word propylaea (propylaeum is the Latin version) is the union of the prefix pro (before or in front of) plus the plural of the Greek pylon or pylaion (gate), meaning literally that which is before the gates, but the word has come to mean simply gate building. The Brandenburg Gate of Berlin and the Propylaea in Munich are specifically copied from the central portion of the Propylaea.

The Pnyx (Greek: Πνύκα) is a hill in central Athens, the capital of Greece. Beginning as early as 507 BC, the Athenians gathered on the Pnyx to host their popular assemblies, thus making the hill one of the earliest and most important sites in the creation of democracy.
https://en.wikipedia.org/wiki/Pnyx

The Parthenon (Greek: Παρθενών) is a temple on the Athenian Acropolis, Greece, dedicated to the maiden goddess Athena, whom the people of Athens considered their patron deity. Its construction began in 447 BC when the Athenian Empire was at the height of its power. It was completed in 438 BC, although decoration of the building continued until 432 BC. It is the most important surviving building of Classical Greece, generally considered the culmination of the development of the Doric order. Its decorative sculptures are considered some of the high points of Greek art.

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.