That, and the technology of crossbows, much less ballistae have been lost to the mists of time. Look at what happened to the Dwemer. Tsk tsk.

And Gcold has the right of it. Just that most of Skyrim's fortifications are simple, like iron-age hillforts with stone walls and wooden palisades. Most of Nordic warfare could be handled with infantry, things like fast and bloody raids. Any walls could've been shouted down, or ladders could scale walls and rams could bust in gates.

Not to mention, Nordic culture and society is borderline anti-intellectual. Power comes from a sword, the two biggest punishments for crime are judicial duels or the chopping block.