 The continual movement of people, rulers, nobles, bishops, abbots, craftsmen and peasants, was an important factor in creating a homogeneity in building methods and a recognizable "Romanesque style", despite regional differences.Life became generally less secure after the Carolingian period.This resulted in the building of castles at strategic points, many of them being constructed as strongholds of the Normans, descendants of the Vikings who invaded northern France under Rollo in 911.