A 28 hectares preventive excavation was conducted in 2011-2012 by the Institut National de Recherches Archéologiques Préventives (Inrap) in Saint-Germain-en-Laye – 30 km north-west of Paris, on the alluvial plain of the river Seine, prior to the modernization of the largest European water treatment plant. This excavation led to the discovery of abundant remains of the fort Saint-Sebastien, built in 1669. Over the course of only 2 years, it allowed the training of siege warfare of 16 000 to 30 000 men, belonging to the military household of Louis XIV. The excavation unveiled impressive fortified structures of the seventeenth century as well as encampment areas within the fortifications with 3 500 archaeological structures and almost 2 km of defensive moats. The excavation of those two field fortifications succeeding within two years for two campaigns puts the siege warfare of the seventeenth century under a unique light. It gives a rare material testimony, in an unusual scale of the military practices and society of the 1670’s and highlight that particular European political context as much as the emergence of a standing army. The data collected has provided precious information about the soldiers and the officer’s daily lives, the origin and forms of supplies of the king’s army and the social and spatial organisation of this military society where infantry, horsemen or musketeers cohabited. Archaeology triggered an investigation mobilizing biological sciences, archives and geography in order to refine the understanding of a complex society of war, never observed in that scale from the material point of view.