This can be difficult as defenders of towns and castles are generally hundreds strong and if you are not a member of a faction, you will not have any allies to help you.
Personal fiefs can be obtained by capturing a town or castle while not a vassal of any lord. There are a few ways of getting hold of fiefs, most of which involve capturing them after a siege. Towns are typically assigned to powerful lords and do not change unless the lord that owned it defects or gets exiled.
Every time you start a new game, the ownership of villages and castles is randomized with only a few exceptions.
