Given that freshly popped units can move out right away, I'd imagine they'd be able to defend a city. I think the bigger question is do reinforcements arrive in time? For example, Stoneheart wants to attack one of your cities, and you're also moving troops to the city, do your troops get there in time to defend the city? Can they get there in time to benefit from the city's defenses? Or will they arrive suddenly having to attempt to recapture a city? Could be a die roll. d6 with 1-2 being you get there late, 3-4 being you get there in time to defend, but can't benefit from city bonuses, and 5-6 you get there in time to benefit from city bonuses. Or maybe just 1 and 6 are special with 2-5 being a strait up fight.