Catherine's Palace  was designed by Bartolomeo Rastrelli, the architect, who created the Winter Palace and the Smolny Cathedral.  Most of the restored interiors date back to the times of Empress Elizabeth, daughter of Peter the Great, though there are some early 19th century interiors too.  Catherine the Great used to live in a separate wing of the palace, and when she was in her sixties, she would still walk through the whole palace on her way to church.