I put Ruthgar fairly far (a good ten minute sprint) away from the city. Mostly because, as Tandi's attempt to enter has proven, they are not going to let anyone who isn't already a part of the city enter, unless they intend to join the guard.
Since there has been no previous remark of a massive off anywhere near the city, and I doubt Tandi would point an arrow at someone who was a part of the caravan, why don't you rewrite your post to move things away from the city? I've been trying to convince everyone that there is no gain to going into or around the city, because none of the plot is going to happen there. You can certainly keep the post you wrote, I'd just change the mention of the undead smashing against the wall to it smashing against... a tree or big rock or even the ground. It may also be worthwhile to bring the fight to an end so that Ruthgar can actually interact with Tandi. I can't say for sure what Tandi would do in that situation because I'm not lynzy, but I'd I had to guess it wouldn't be "help the giant, crazy, PTSD inducing Orc kill even more of the undead".
Of course, all of these are just suggestions. You don't have to move your character, and I suppose it is possible Tandi could have gotten turned around while fleeing. But there's no reason to stay near the city.