Soaring 80 feet high, the Marble Hall at Mount Stuart is a stunning blend of rare Italian and Sicilian marble, described by Gavin Stamp as the "dark and mysterious heart" of the house. Inspired by Aachen Cathedral, which mesmerized the 3rd Marquess of Bute as a child, the hall reflects his fascination with astrology. Stained glass zodiac symbols and a vaulted ceiling filled with stars create a celestial wonder.

Though structurally complete by 1884, much of the hall's decoration was unfinished when Bute died in 1900. His zodiac-inspired marble statues were installed after his death, placed based on a 'best guess.'