Vikings certainly traveled by coast but attacked towns farther inland as well. A town might be aware of Vikings near by based on news from neighboring towns or by sighting viking visitors on the coasts. The frequency of attacks were more based on accessibility of the town and its resources. If you had rich farmland, were located at a key trading point, or were just pretty well off, you would be at a greater risk of having viking guests. The time of the Vikings went on for several hundred years, so their movement was more a gradual spread than a continent wide attack. A greater awareness of Vikings were known, many towns were even controlled and inhabited by them, through trade routes and migration.