Uriel, just doing some thinking aloud.
Are there any troops of value still outside the gates? Taking the gatehouse only makes sence if there is something left to be let in. Originally, that was us, but now we are inside. But if there is nothing much to come in, then it may make sence to set another objective. We're here to find/free the sha-man who can bring back Zugerath. So moving towards where Durgo and Co are likely to emerge, or getting further inside the castle proper come to mind as options (I am kind of stabbing in the dark here as I don't know anything about the castle layout). Where is the Sha-man likely to be? Down below? In a tower? Anywhere?
Of course, Durgo and co don't know that we are inside, so if it likely that we would wonder around the castle for ages looking for each other, then it may be best to take the gatehouse and hold and wait. How far from the gatehouse are they likely to emerge, and what would be the most direct route from there to the gatehouse?