[X] Fortify the road to Kislev to keep the Daemons isolated from population centers.
So, the plan's a little riskier than the other fort up plans if the Daemons play it really smart—exit, group up, and head our direction (aka the only one we care about). Thing is, they're Daemons, and coming off of a large strategic loss at that. I don't expect them to be particularly smart about things.
If we're sitting right outside, we will be a convenient and obvious target to vent their frustration over their whole thing going tits up at. They'll know we're to blame, they'll know we're right there, and it will not be hard for them to point the others at us.
If we're a ways down the road in the only direction that leads to civilization in a time frame that they can last, we're only dealing with the bulk of their forces if they decide to leave the Karak, if they remain under somewhat unified leadership, and if said leader either gets lucky in pick a direction or actually knows the local layout and travel times (which is far from a given given Daemon).
Basically, we avoid picking a fight needlessly with this option while if the fight comes to us not actually having much worse of a setup than doing so right outside the Karak.