Honestly, I'm sort of at a loss on how to approach the objective. Maybe... find a way to tunnel? Throw up a disguise? Cause a distraction somewhere else so we can sneak by without attracting too much attention? Find someone else inside the city who's not an enemy and who knows the city better and can help us?
Then, there are the guards at the city entrance who must be dealt with once we decide how to approach, right?