The urban shrinkage and the fortified city
 
  Urban shrinkage
Between the mid 3rd and the late 4th century, Lutetia's left bank grew increasingly unpopular. This can be seen in the abandonment of the Early Roman necropolis and the appearance of small groups of graves in the city itself, which is contrary to the classical separation of the worlds of the living and the dead. Most of the city's public monuments were abandoned and systematically stripped of their stones. Habitation areas were partially deserted and small house stones were reused in other buildings.

The fortified city
The 4th century construction of a castrum on the île de la Cité ended this process, breaking with the concept of the large, open city of the Pax Romana.
  This transformation is typical for most of Gaul's major cities during this period of instability. Lutetia became a defensive stronghold in the struggle against the Germanic invasions. On several occasions, the city became the residence of the military emperors Valentinian and Julian-the latter being proclaimed emperor by his troops in 360 CE.
However, we should not think that the entire population of Lutetia lived within the perimeter of the wall, which enclosed an area of less than 10 hectares. The left bank was not entirely abandoned. According to Roman historian Ammianus Marcellinus-as well as observations made along the cardo and in Lutetia's abandoned, pillaged monuments-people continued to live in the city's inner suburbs and in a number of insulae on the north face of the Sainte-Geneviève hill.