[Arryn]

Originally posted by PvK:
Agreed, though what happens if you re-take the province the turn you try to build troops at a besieged location?

Well, the castle was beseiged. The defenders would have a tough time getting a messenger out, nevermind not having him caught while trying to recruit in territory that's been overrun by an invading army. It's just a wee bit unrealistic, if you'll pardon my using that term. While messengers escaping from beseiged keeps did occur historically, it was typically uncommon, and mostly happened with combinations of inept attackers and bad weather.