I don't know about trenches, but I do know in one video they show that before a battle there are certain defenses you can place, such as small fences and the like. So I doubt you'd see your men actively digging a trench but a trench may be able to be created before a battle.

I think you can attack a city, as I recall one video showing a army blowing through a city wall, but I do believe the article said most cities you have to get through a fort.

Remeber this time armies have circles, so this isn't a issue of your units meeting at a exact point. So I assume as long as fort is in radius of a city you'll have to take it to take the city, though I could be wrong. That's just my speculation.