Fortress Refuge

The more I write about this area, the more I like it as a spot to stick with for an extended time. There is so much that can be done in this setting. Once the tunnels and dungeon are cleared, the spot would be an excellent location to rest and recover in. It would allow for a number of encounters involving groups attacking the fortress with the party using it from a defensive position.
I think a white dragon in the hills would at least do a flyby or two, if not an all out attack. There are numerous wolves and winter wolves in the mountains that would assault. In addition, there may be additional cult members caught out during the original attack returning, or just coming on missions or with direction to retake the hill.
It might be kind of fun to have the white dragon be a young dragon establishing a lair in the hills. The party is aware of this, and is inclined to seek it out and kill it before it can gain a foothold or more power. This could let the DM touch on some of the lair attributes in anticipation of the main dragon approach at the end of the adventure, but due to the youth and weakness of the dragon (because of it recent arrival), the effects would be partial or weak. In game terms, this would mean the party is aware of the presence of a dragon and the likelihood of a lair, but would need to make multiple forays into the hills to hone in on its location. This is perfect for several random, different, arctic/mountain related encounters - with bears, wolves, remorrhaz, or even a frost giant.
Though giants and dragons are typically rivals, the dragon may be establishing a lair in territory inhabited or traveled by giants. If you wanted a particularly nasty three-way battle, start a fight with a frost giant and have the dragon swoop down to join in. Both monsters would make impromptu "allies" against the other, but neither would be a friend to celebrate victory with...
One way to entice the party to remain for a period of time would be to have them here to find a frost battleaxe to use against the red dragon later. My original thought was that the item would be in the custody of the cultists. A better way to explain everything would be that they came here seeking it, and enlisted the kobolds to excavate the collapsed areas of the dungeon, but to no avail. Instead, they found references to it, and evidence that much of the fortress and dungeons have been recently looted. They suspect that the white dragon frequently seen overhead scoured the ruins for treasure to help her establish her hoard. They had begun to investigate where her lair might be, and how to find and best use the weapon. A captured prisoner would reveal this information with the proper persuading, otherwise it could be gleaned by careful study of the sorcerers' notes.
There is a good opportunity for some skill checks tracking things in the snow, as well as dealing with icy slopes and cliffs, exploring fragile snow caves, and avoiding avalanche. Actually, having a blizzard hit and snow them in might be the perfect way to keep them in the fortress. This would give them time to search the ruins, and really study the sorcerers' notes. Reward successful searches with interesting equipment or treasure in the form of art objects uncovered in the rubble. From the sorcerers' rooms, the treasure found would be magical in the form of scrolls and potions.
With the valley snowed in, the tunnels would become attractive refuge for wild animals in the region, giving options for encounters with a variety of natural critters after food, shelter, or dominance of the territory. Encounters could come from the cave and tunnels, or the buildings of the fortress above. In some ways, the party could be trapped with aggression approaching from two separate directions.