The folks at Warhammer Online have posted a guide to city conquest, where they explain the process of City Sieges in the game.

The culmination of the ever-present conflict in Warhammer Online is the capture and ransacking of the enemy’s capital city. Herein we’ll detail the steps necessary for opening the gates of the city, and ultimately taking it for your realm. Later on, we’ll have more specific guides for each step along the way, but every razing and pillaging group of warriors must start somewhere, so let’s begin with the basics.

Opening up the gates of either city is a long and arduous process that begins in the Tier 4 zones primarily. In order to force your enemy’s city into the contested state, you and the fellow warriors of your realm must first capture 2 fortresses in the Tier 4 zones within 12 hours of each other.

In order to attack these fortresses, you must first capture the zones prior to the Fortress zones in Tier 4 by Zone Control (participating in Scenarios, queuing for Scenarios, capturing Objectives and Keeps, Killing enemy players and completing PQs). What this means is you’ll first push to a zone between your Tier 4 home and the enemy’s (Praag for example), and once you’ve taken control of that zone you would push forward to Reikland or Chaos Wastes (depending on your allegiance) and capture the objectives there.

Read more here.

Are there any areas that a side can take over and keep it until they are dethroned?  I have not played Warhammer yet but it seems if you went through all the trouble of taking a city over it would suck to just give it back after 20-24 hours without a fight.  Or does it make sense to do this with how the game is set up?  Just curious.  I have been following this game with intentions of maybe playing it at some point.

Most areas are like this..they do have a feature where they reset due to maintence, patches, etc or your guild runs out of money to maintain the keep.  THere are reset features there for good reason but for the most part most keeps change ownership because of players not a game mechanic except Tier 4.    They do not just how oever throw you out of the city eventually the  city's defenses such as NPC's etc pretty much force you out if the players are unable to do it so you do techincally fight.  Once they force most people out and the timer is up it does 'reset.  Otherwise realms would be completely dominated due to population imbalances or timing.