Actually, I tried running up to it, but it still didn't open. What worked was double clicking on it. Either way, thanks to your response I went back and figured it out.
Most people have auto open doors enabled in razor, takes a bit getting used to sometimes but it can be a life saver in many situations. Holding a spell or command and going through a door for example. Anyway if you enable that option in razor then the gate to castle will open with no delay anytime you run through it.