The first dressed stone structures appeared on the heights of this part of southern Oise during the Celtic era.
The stone fortifications of the Gauls were conquered by the Roman invaders and subsequently re-used.
The Roman occupation (49 B.C.E. to 275 C.E.) witnessed the first large solid stone constructions. The Roman town of LITANOBRIGA, which later became SAINT-MAXIMIN, has numerous vestiges of stone construction: the quay of a port opposite Saint-Leu, a stonemason’s yard, and lots of villas.
Stone was quarried from SAINT-LEU (right bank) and TROSSY (left bank). It was transported by water as far as PARIS (architraves of the ROMAN FORUM in PARIS).
A stonemasons’ village existed at Saint-Leu-d’Esserent between the 1st and 4th centuries C.E.
Between the 4th and 9th centuries C.E the area was devastated by the Barbarian invasions which followed the Gallo-Roman era.
A period of stability returned with the arrival of the first Merovingian kings.