I found a way to siege towns using this "cheat" dealio... (!)
Don't blow chunks until you try it. I took Praven as a Nord.
Procedure:
1. Show up to an enemy town with your party... (haven't tried with friendly towns)
2. Choose CHEAT! External
3. Walk around in the city until you get to the keep, and use the action key when the door prompt shows up.
.
ok
->Press F, or your otherwise assigned action key.
4.You should lag for a second, then respawn in the town with a couple of your troops. No enemy troops spawn, however.
5.Press tab, and select yes, you do wish to retreat.
6.Typical siege screen, construct the damned ladders, etc..
7.Go charge the enemy, do the whole thing.
8.Win.
Unfortunately, since the city map was not designed to be sieged, enemies tend to spawn in... odd places. If you intend to complete the siege in person, bring a bow and a cheat code.