I was wondering if at the end of the Imperial Legion/Stormcloak quest-lines what happens to these cities? I've heard one gets destroyed but in what respects? Does it get fixed, do the walls just look damaged? Or do the cities themselves actually become void from the game?

Goes back to normal. I just finished the Civil war quests on the stormcloaks side and every npc in Solitude seems to still think the imperials STILL own the city... half ass if u ask me but everything goes back to normal.