I read(1) that only the northern part of Seoul the city is in striking distance of their (somewhat limited in numbers) long range artillery, and that there are lots of shelters e.g. the way the subway system doubles up as being bunkers.

However the civilians part between Seoul and the DMZ, home to millions, is not going to fare exceedingly well - if the North decides to "waste" their ammo on civilians instead of the military that will be hitting them back.

1) https://www.quora.com/Can-North-Korean-artillery-really-kill...

Indeed, it is hard to overstate the damage NK could wreak on Seoul, even in the opening hours of a full scale conflict. Roughly half of the population of SK lives in the greater Seoul metro area. Artillery barages could claim millions of lives, and wreak economic devastation, and we're not even talking about nukes yet. I don't relish the thought of Kim, backed into a corner, fearing total oblivion, his fingers fluttering inches away from the big red button.

I suspect that the US has had the capability to disable ICBMs for some time now. (See patriot missile system and iron dome and remember Regan's star wars project and its not a big leap to make) I think it's just a matter that we don't want to let would be nuclear assailants know until they have already committed.