Found this interesting, not ever having seen it before. This is an excerpt from Tobold's blog.
You have to keep in mind that a City capture is a long multi-stage process that requires all of the following elements (I will use Aldorf as my example).
1) You must capture two fortresses in Tier IV within 12 hours of each other. To gain access to a fortress you must seize and hold the neutral Tier IV zone and your opponent's zone in two separate racial pairings. Once you lock down one racial pairing, you only have 60 minutes to capture its fortress. Should you fail, then that racial pairing reverts back to a neutral state and you must begin anew.
2) You must then fight in contested Altdorf and gain control much like you would in a zone. There are two Battle Objectives, a Public Quest and opposing players you must contend with in order to accomplish this.
3) Should you accomplish the first two tasks, the opposing realm is ejected from the city. At this point you have one hour to cement your hold on the city. You must complete the Temple of Sigmar and Bright Wizard College Public Quests within the next hour. If you manage this feat within one hour, you then lock down the city for 24 hours and unlock the King. If you fail, then you have one hour before the city is re-opened to opposing Realm players.
4) If you defeat the King, then you can unequivocally say that you captured Altdorf.