In any case, one similarity that Skyrim and Hammerfell have is that their climates and terrains are very harsh to outsiders, and only the locals are uniquely suited to thrive in them. That's the biggest reason why I believe that the Nords would be able to defend Skyrim the way that the Redguards defended Hammerfell. And contrast both lands with Cyrodil, which, if you've played Oblivion, you'll recall has a fairly temperate climate and terrain mostly consisting of grasslands and forests. Not an easy place to defend from invaders.