It is well to bear in mind that after the death of Charlemagne his empire gradually fell apart. A German empire extended from the Rhine to the Danube and was in constant conflict with the Popes to exert its sway over Northern Italy; the growth of the communes or free cities being perpetually disturbed by siding with one or other of the contestants—the Imperial or Ghibelline and the Papal or Guelph.
France, meanwhile, was not yet a united nation. The kings of the House of Capet held only the so-called Ile de France or Royal Demesne, extending from Paris to Orleans, and were surrounded on all sides by independent Duchies and Countships, with which they were constantly at war. The Duchy of Normandy had been established to the north by Rollo and in 1066 his descendant, William, conquered England.
These distinctions of territory help to explain the variations of the Romanesque architecture, as it grew up, respectively, in Northern Italy, the Rhine Provinces, Ile de France, Southern France, Normandy, and Norman England.
PISA CATHEDRAL, CAMPANILE AND BAPTISTRY
INTERIOR OF PISA CATHEDRAL