EXPLOIT: You can use neutral armys to "steal" citys.

[autotron]

When you see an army that is neutral towards you on it's way to besiege an enemy city you can position yourself infront of the city with a small group of soldiers and wait until the army is next to you. When you then start a siege the city will immediately sally out because you are so weak but will pull the army into the fight on your side. You can then let the army defeat the militia in a field battle and take the city for yourself because you initiated the siege.

 

[Ackdam]

Sounds like tactics though should probably piss off the noble who intended to attack...I could see them working with this though. Think, joining another faction's siege, then go through a series of checks to claim that your party/army did the most work and thus you deserve the fief. Possibly add in a bartering option to get them to agree and shove off.