The Church of the Holy Sepulchre is a church in the Christian Quarter of the Old City of Jerusalem.

Heilbronn is a city in northern Baden-Württemberg, Germany. It is surrounded by Heilbronn District and, with approximately 123,000 residents, it is the sixth-largest city in the state.

May 5 or May 6 Monday – The Clemens family arrived in Heidelberg, Germany and stayed at the beautiful Schloss Hotel, which overlooked the old castle with its forest setting, the flowing Neckar River, and the distant valley of the Rhine. https://daybyday.marktwainstudies.com/vol-1-section-0036/ Vol 1 Section 0036 – Mark Twain Day By Day

The family returned to Heidelberg for a week in August and September of 1891, before moving to Switzerland.

See Bædeker The Rhine from Rotterdam to Constance, 1873 page 176

The Quaker City passed through the archipelago twice, first on its way to Constantinople from Greece and again returning from the Black Sea en route to Smyrna.  Twain remarks on passing between Lemnos and Mytilene.

The Quaker City sailed through the Greek Islands August 13-17, 1867.  It stopped at the harbor of Athens August 14-15, during which Twain broke quarantine to visit the Acropolis.  

Isaac Gewirtz, in Mark Twain A Skeptic's Progress, writes of Twain's view of the French Revolution:

On looking at the map, this location seems to actually correspond to Ayassalook, where the Quaker City party got off the train and took donkeys to Ephesus.

Ephesus was one of the great cities of the eastern Mediterranean during Roman times.  It contained the Temple of Diana which was considered one of the Seven Wonders of the World.

Near Eberbach there is a crumbling ruin known as the "Spectacular Ruin".  The raft captain tells a tale about a dragon that terrorized the region. A Sir Wissenschaft killed the dragon with a fire extinguisher and was granted a spectacles concession by the emperor for his heroism.