Nords, of course. Fierce, armored, bearded men with large shields and two-handed axes. And one-handed axes. Hell, they even throw the things. Ragnar talks about the right to rule going to the strongest, and his armies, if left unchecked, will usually destroy Swadia completely and start to menace the Vaegirs and Rhodoks. I hate the Nords for always destroying Swadia, but you gotta give them some credit for almost always being the winners. In sieges, they're usually both an unstoppable force and an immovable object.