Ok.. so you start the civil war questline, you have a few interesting quests to start, then it just turns into a garbage version of Mount & Blade with re spawning enemies in the forts.
They aren't quests, they are bonus combat rounds for free xp. See how many you can kill for extra points (and weapon skill) or let them beat on you (to train armor / block skills).
I agree though, it would have been cool if the later forts had something besides gumpy soldiers. Maybe the opposition gets a soldier (or whole platoon) that gets the essential tag and turns into a (non-essential, full health) werewolf when at 0 health, for example. Or maybe they convince a giant to help defend the fort in exchange for free food. And they build him some giant scale armor and magic items. Or all the defenders have been turned into vampires by their desperate leader in a last ditch attempt at defense, and won't come outside during the day, forcing you to clear the interior of the fort vs respawning vampires.
I did have one "fort capture" mission early on where a dragon landed in the middle of the fight. That was VERY cool, and I kinda expected similar things later on.