In your Riverlands development plan do you think it would be a good idea to try to seize one of the twins castles and give it to another house to weakening the Freys and use at time to build another bridge in the green fork.

Seizing one of the Twins would be a difficult and inelegant approach – better to wait for Walder to die and when the civil war starts up, let them weaken themselves (by consistently supporting the weaker side in the conflict) to the point where you can take the Twins outright, put a cadet branch of the Tullys up there, and then let the Rootes or Vyprens build a bridge.