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

See Bædeker The Rhine from Rotterdam to Constance, 1878 page 204