Hmm. Magic is probably preventing that from happening. Still, pretty dumb ass idea to build a city up there.
could say the same about the settlement atop Machu Picchu...fact of the matter is if anyone marched on Solitude / machu picchu they would not have a hope in hell of taking it.
providing there's an arcane barrier around Solitudes weak points there not going anywhere the people marching on them would just drop like flies, or rather ant having boiled water poured over them, its a pretty smart choice really considering theres magic in use aswell in the game world im sure there protected from any collapse.
on that note though i doubt any thought of the defences etc went into it i bet someones head exploaded with that idea when they where writting there lore whenever that was.
i hope it doesnt get destoryed! i love it!