The Dark Circle proper is the area at the lower right... the undead army is actually forward of that position, trying to expand the circle. The Dark Wall (as opposed to Circle -- located NW of the Circle) is actually a partial line of fortifications and troops belonging to the Duke. This is a sizable army, and current thought is that the undead legions will be directed around it -- anything that doesn't involve desecrating holy sites, graveyards, and places of power isn't on their first agenda. Swinging right isn't much of an option... it's too well defended and the undead wouldn't have a bridge crossing over a large and deep river. On the other side they'd have swamps and druid-controlled forested hills to pass through, and the losses would be great. They'll probably end up swinging left, trying to wedge into the gap between the unassailable Regar's Keep and the bridge over the Rithe Regar. If they can take the latter they will, if not they'll try to sack Southgate, which is not quite as well defended as the Keep, and that will give them the river crossing they need.

From there they pretty much can't be contained again until they are constrained at one of the bridges over the Sweetwater. That's where you guys come in.