How could you have been saved from the Viking invasion?

The harsh natives of the Scandinavian lands were a real nightmare for the inhabitants of Europe at that time. Endless looting, murder and chaos forced Europeans to think about how to stop the Viking raids. For this, they began to block the river beds with chains (so that the drakkars could not swim); transferred residents from small coastal villages to large cities, built forts and fortifications there; created cavalry detachments to quickly confront the Vikings who had recently landed on the coast; began to build ships with high sides and flamethrower siphons.