From 1782 until 1789, the princesse de Monaco (1737-1813) and her lover the prince de Condé (1740-1818) created at Betz (Oise) one of the most costly and ambitious landscape gardens of pre-Revolutionary France. At the heart of Betz was the ‘Vieux château’, a pastiche ruin composed of authentic architectural and sculptural elements salvaged from the demolition of medieval buildings. Through this architectural set-piece, Monaco and Condé recounted to visitors the tragic tale of a virtuous medieval couple, whose mutual-devotion and unfortunate fate mirrored their own controversial love affair. Following her separation by act of the Parlement de Paris from Honoré III de Grimalid, prince de Monaco (1720-1795) in 1770, the princesse was forbidden from attending court or sharing the official residences of her lover Condé — a royal command that the couple flouted openly. The article explores the design and development of the garden through the princesse’s household accounts, now conserved at the Palace Archives of Monaco. It examines the role that the landscape painter Hubert Robert (1733-1808) reputedly played in assisting the couple with the design of the ‘Vieux château’ and proposes that the architect Alexandre-Théodore Brongniart (1739-1813) also contributed with his expertise to the project.