This town has a inherited an ancient past and is where the Saint-Omer region first originated.
Thérouanne was already the capital of the Morini, a people of Belgian Gaul. After it was conquered by Caesar, it became a crossroads of Roman roads and a major town developed there. After multiple invasions by Germanic tribes, it became the seat of one of the most powerful bishoprics when Omer was appointed its first bishop in 683.
Several cathedrals were built there one after the other and walls were built around the city. Charles V of Spain besieged the city which was a French enclave in the Spanish Netherlands, and had it completely rased to the ground in 1553, thus creating one of the rare examples of “dead towns” in Europe and a remarkable archaeological site.
Beside the site of the ancient city, the village of the same name grew up.
The outline of the ancient city can still be seen in the landscape. In its centre, the site of the old cathedral is open to the public. In the village, the archaeological museum presents the fifteen centuries of the town’s history through its Gallo-Roman and mediaeval collections of objects found in the hundred-odd digs that have taken place at the site. We also get a glimpse of the everyday lives of the people who lived there.
Thérouanne also extended to some outlying districts, Nielles along the riverside being one of the best preserved of these. The chalk of the sub-soil was used to build the old chapel and farmhouses. The town and surrounding countryside can be seen from one of the many footpaths that pass by the area.