Originally Posted by Josh Warner
Armies can merge freely.
As far as multiple armies in a city, it's not really finalized since it's a balance issue not a mechanics issue. We'll see how that ends up working come beta. Again though - armies are going to have a max number of battalions in them. A 'full' army won't be enough to reach the pop cap. So you could easily have one army off doing something in one corner of the world maybe they're exploring the caves in the dwarf region, another army with your hero doing a quest attacking an Orc NPC city and a third army defending your city. Etc.
Will you only have one hero, and your armies that move across the map don't have to have a hero leading them, right? If you only have one hero, will you have the same as everyone elses hero who are the race as you, or is there a choice?
As for the quest to attack player cities, it reminds me of bounty quests and the like. Where someone would send out a reward for killing a person, there could be an allotted amount of days that they would have to complete the task, and then they would receive a reward, like resources and/or possibly some units.
But one point for quests just being sent out for people to attack you (what was discussed on the first page) would be to have an option to participate in quests like "I accept being able to be attacked by fellow players for a quest." that they can turn off at will, but not during a battle for the quest. Another thing I would like to add is that an AI general should control your army while your offline and are attacked for this quest. It should also not affect your city so you don't log back on and find your city half dead or some other such catastrophe.