Le Châtelet de Gourzon, Archaeological site in Bayard-sur-Marne, France.

Le Châtelet de Gourzon is a forested plateau of about 24 hectares above the Marne valley at Bayard-sur-Marne, enclosed by an irregular looped rampart. Excavations since 1772 uncovered a Gallo-Roman secondary town of the Leuques, with a fanum whose cella measures about 65 m², hypocaust-heated baths, houses with courtyards and wells, and a craft quarter of potters and smiths. Only a few hectares have been cleared; the rest lies under trees.

The plateau was occupied from the Neolithic and Bronze Age, then continuously from the Iron Age, when the Gallic Leuques made it an oppidum controlling the Marne valley, to the High Middle Ages. In the Roman period a secondary town of about 24 hectares covered the whole plateau, near Nasium. From 1772 to 1774 Pierre-Clément Grignon dug there methodically, with prior soundings and detailed reports, funded by Louis XV: the first subsidised excavations in France.

The name Gourzon survives in the hamlet at the foot of the plateau, while the engraved 18th-century plan labels the site "Montagne de Chatelet" and marks "R. Ruine artificielle" among its key. The Gaulish oppidum of the Leuques stood near the borders of the Remi and Lingones and, judging by its coin finds, served as a trading centre controlling movement through the Marne valley.

The plateau is wooded and the excavated remains are largely overgrown, so there is little built structure to walk through; the Roman aqueduct outlet near the road toward Nasium, restored and listed in 1883, is the most visible piece. The plateau can only be entered at two points, west and east. A Lidar survey is under way, so parts of the site may be closed off.

The water works east of the town, along the road to Nasium, were long called an aqueduct, but recent research shows they are a qanat: a buried conduit tapping the water table, with service shafts every 6.21 metres. Its heavily restored outlet can still be seen, and the remains were listed as a monument historique in 1883.