The castle hotel in Heidelberg was from 1873 to 1964 intermittently a hotel . In a location-dominant location above the Hortus Palatinus of the Heidelberg Castle , it was once one of the best houses on the square, which accommodated numerous prominent guests. Even Empress Sissi , Richard Wagner and Mark Twain were guests here. After the end of the hotel business, the building served until 2000 as an international study center of the University of Heidelberg . Since 2009, a residential complex of apartment complexes has been built on the site of the building.
From Wikipedia (link errors)

*SCHLOSS HOTEL. And PENSION, with fine view, R. from 3.#, L. 50, A. 50 pf., omnibus to meet the trains, cab from station 3.# 50 pf.  

Bædeker The Rhine from Rotterdam to Constance  (1878)

Bædeker's 1886 edition of this handbook does not mention the Schloss Hotel, although it does appear on his included map.

Now if one turns and looks up the gorge once more, he will see the Schloss Hotel on the right perched on a precipice overlooking the Neckar—a precipice which is so sumptuously cushioned and draped with foliage that no glimpse of the rock appears. The building seems very airily situated. It has the appearance of being on a shelf half-way up the wooded mountainside; and as it is remote and isolated, and very white, it makes a strong mark against the lofty leafy rampart at its back.
This hotel had a feature which was a decided novelty, and one which might be adopted with advantage by any house which is perched in a commanding situation. This feature may be described as a series of glass-enclosed parlors CLINGING TO THE OUTSIDE OF THE HOUSE, one against each and every bed-chamber and drawing-room. They are like long, narrow, high-ceiled bird-cages hung against the building. My room was a corner room, and had two of these things, a north one and a west one.