A city will, at the beginning of a server begin as a Poor city, and may eventually become Prosperous. This is considered the city's Rank. Currently, cities can attain ranks from 1 to 5, although the names and benefits of these ranks are mostly unknown. Leveling to rank 5 is estimated to take a significant amount of time (months not weeks).
The rank of a capital city is proportional to the success of the players. City rank contributions are determined by what zone players adventure in, not by their army. A city's rank is tied into WAR's campaign system, as well as PvE achievements, and other factors such as crafting. The more you achieve, the richer your city becomes; gaining victory points in the campaign and successfully completing PvE quests all helps to create a thriving economy, causing capitals to become more prosperous. This will change the look of the city (a poor city might mostly be a slum covered in garbage, while a prosperous city will mostly be rich & clean neighborhoods). City guards will also level up with your city, so at rank 1 a guard will be a peasant drafted into the army and at rank 5 you will have well armored knights defending the city. Additional quests and features within the city will also open up when your city gains rank. For example, while Altdorf is poor, a player may overhear a peasant talking about seeing a giant "rat man"; as the city becomes more and more prosperous the culmination of the story will reveal Skaven tunnels under the city, allowing for players to explore new territory.
In addition to providing friendly players with special events, and new quests, prosperous cities are a source of wealth and opportunity to enemy looters. Capturing a city allows the attackers to take part in the events and quests that are available within the city, meaning that rank 5 cities are a much more valuable target to the opposing realm. By opening up the World Map, a player can then see the rank of a city, and see what events are taking place in the city at this time. This allows for greater coordination among players of a realm for offensive pushes in the campaign.