Belgium has been aptly called "the crossroads of Europe." Its tribes, which were probably Celtic, were colonized by the Romans for the first three centuries of the first millennium. In the fourth and fifth centuries the Franks invaded from the north. Their settlement was sparse in Wallonia, which ensured the continuation of Romance dialects. The city of Liège, the largest city and cultural capital of Wallonia, was designated as a Catholic see in the seventh century. Under Carolingian suzerainty ( A.D. 752-918), it became an international center of learning. It then became the seat of a large independent principality and evolved into an important city during the late Middle Ages and early modern period. Burgundian dukes annexed Wallonia and Flanders at the end of the fourteenth century, except for the principality of Liège. The Burgundian territories were passed on to Spain and then Austria. In 1789 the people of Liège drove out their leader, the Prince Bishop, and together with the rest of Belgium joined the Republic of France in 1795. In 1815 Belgium fell under the leadership of William I of Holland. This only lasted until 1830, when Belgium revolted and won her Independence. Since this time there has been conflict over linguistic rights within the state.