Charles the Simple, the King of France, managed to defeat Rollo at Chartres, but realised that the Vikings would be back. So he granted Rollo the land of Normandy in 911, on the promise that Rollo would defend the coast and convert to Christianity. And by and large that’s what Rollo did.
The settlement of Normandy was not heavy, and was focussed around the coast, as the map shows. Rollo and his successors encouraged the Vikings to go native. Within a hundred years, the Normans were as French as anyone.
