Most medieval defenses were built around a simple assumption: danger comes from the coast or the roads. Put watchmen on headlands. Fortify the gates. Patrol the main routes. Rivers, meanwhile, were treated as lifelines—for trade, fishing, ferries, and everyday movement. The Vikings looked at the same rivers and saw something colder: highways into the heart of a country. The longship made that possible. Long, narrow, and relatively light, it was built with overlapping planks (clinker construction