Hau made this set of 111 watercolors of the Royal living quarters during the 1850s-1870s. They can be viewed at two different sites:
Feeling skeptical? Try just this one painting of the Antechamber of the Tsesarevich Alexander Nikolayevich. (Be sure to check out the zoom.)
Complete set of 111 drawings (official Hermitage site)
Complete set. This site has far the superior presentation.