Mont César, Protected natural area in Bailleul-sur-Thérain, France.

Mont César is a hill in Bailleul-sur-Thérain, in the Oise department, rising to 138 metres on the left bank of the Thérain near the hamlet of Froidmont. Its lower slope carries rows of low leafy crops, a band of flat crop lies above them, and dry pale grass covers the upper slope up to a scattered line of trees near the rounded summit. Most of the hill is wooded, and its dry calcareous grassland, a larris in Picard, is among the best preserved in Picardy.

The hill was occupied from the Bronze Age and likely became a Gallo-Roman oppidum built on an earlier Gallic fortified camp. Excavations from the 18th century onward uncovered numerous objects and remains of fortifications; in 1878 Isidore Berton found Gallic coins and Roman-era items. In 1909 Victor Leblond placed Bratuspantium, the oppidum of the Bellovaci, on Mont César.

The name Mont César was attached to the hill by tradition, and in 1909 Dr Victor Leblond, president of the Société académique de l'Oise, placed the Bellovaci oppidum of Bratuspantium here. The summit now holds a statue of Notre-Dame du Mont-César, and the hill remains a popular walking spot reached by a footpath.

A hiking route winds through varied landscapes with grasslands, marshes, and agricultural fields that takes several hours to explore. Visitors should wear sturdy footwear and be prepared for different ground conditions.

The dry, calcareous larris of Mont César is one of the best-preserved in Picardy, and it shelters a rich set of wild orchids: the leafless limodore, the pyramidal orchis, the military orchis and the bee ophrys. The site is part of the Hez-Froidmont forest massif, listed as Natura 2000 site FR2200377 in March 1999 over 852 hectares.