Head to East Wall.
Taking a route through the nearby trees, both Noxia and yourself manage to sneak under the knight's notice. Under a short tower, you find the hole that the old man spoke of.
Noxia: "I think if we go far enough, we'll be able to reach the middle of the city and then reach our objective from there."
Gordan: "Hopefully Marcus has already found Francis."
Noxia: "Who's that?"
Gordan: "A friend I met along the way, haven't seen him since yesterday."
Dropping underneath the stone entrance, you stumble upon a passage lit with torches. It doesn't seem like this place is abandoned. That, or the torches have been burning for a ridiculously long time.
Your map has updated. It seems that it would take at least four rooms to get to the center if you take the passage to the left. The right seems to head towards a waterway.