+1. I agree. If not all of the city then give different sections that they assault per instance. Say, if they win the walls, then you fall back to the inner city, you then are put back to the campaign map to decide whether you will surrender or try further defend a inner portion of the city. This ensures optimization, and extends gameplay. Then a new battle starts elsewhere in the settlement if you choose to go on. You should have the ability to lead or send troops too.