This proposal originates from the ANR TransMonDyn project, which aims at analysing settlements systems evolution over the long time using modelling. This project gathers many case-studies, and I plan on presenting one of those, that aims at a better understanding of the social and spatial processes that happened in NorthWestern Europe between 800 and 1100 A.D. Through this period, archaeologists and historians (TANNIER & al., 2014) observed a major change in the settlement structure, to which we refer as the " 800-1100 transition " :-The peasant households, mostly sprawled around 800, tend to cluster around functional attractors, mainly castles and churches. Furthermore, they cluster for a longer time than during previous periods, and this clustering and fixation are at the origin of villages and small towns.-These aggregates of population and the simultaneous Gregorian Reform lead to a stronger religious control over the peasants, and results in the appearance of a regular grid of parishes designed around the churches. This well-defined and structured territorial meshing is at the origin of current administrative organisation.