Did you know about Gustave Eiffel's "Secret" Apartment atop the Eiffel Tower? It is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous. It is located on the third level of the tower. In Eiffel's time, the small room was filled with wooden furniture, colorful patterned wallpaper, and a grand piano. The apartment also held a small laboratory area, which the designer outfitted with the time's most high-tech scientific equipment. It wasn't that "secret". It was one of the conditions of Mr. Eiffel before he took up the project of construction of the tower.
Source: Atlas Obscura & Deepak Jakkareddy
7ème Arrondissement Paris