The domed churches of Constantinople and Eastern Europe were to greatly affect the architecture of certain towns, particularly through trade and through the Crusades. The most notable single building that demonstrates this is St Mark's Basilica, Venice, but there are many lesser-known examples, particularly in France, such as the church of Saint-Front, Périgueux and Angoulême Cathedral. Much of Europe was affected by feudalism in which peasants held tenure from local rulers over the land that they farmed in exchange for military service. The result of this was that they could be called upon, not only for local and regional spats, but to follow their lord to travel across Europe to the Crusades, if they were required to do so. The Crusades, 1095–1270, brought about a very large movement of people and, with them, ideas and trade skills, particularly those involved in the building of fortifications and the metal working needed for the provision of arms, which was also applied to the fitting and decoration of buildings. 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. Political struggles also resulted in the fortification of many towns, or the rebuilding and strengthening of walls that remained from the Roman period. One of the most notable surviving fortifications is that of the city of Carcassonne. The enclosure of towns brought about a lack of living space within the walls, and resulted in a style of town house that was tall and narrow, often surrounding communal courtyards, as at San Gimignano in Tuscany. In Germany, the Holy Roman Emperors built a number of residences, fortified, but essentially palaces rather than castles, at strategic points and on trade routes. The Imperial Palace of Goslar (heavily restored in the 19th century) was built in the early 11th century by Otto III and Henry III, while the ruined Palace at Gelnhausen was received by Frederick Barbarossa prior to 1170. The movement of people and armies also brought about the building of bridges, some of which have survived, including the 12th-century bridge at Besalú, Catalonia, the 11th-century Puente de la Reina, Navarre and the Pont-Saint-Bénézet, Avignon.