The North doesn't have the capacity to lauch a nuclear missle. Any other missle launch would have to be part of a first strike by the Norks and I think there is zero chance of that. And even if they did, I think the Aegis system could take them out. That leaves artillery which is a problem, but a manageable one.
BUT... even if you're right. Let's say no missiles can get through. The Norks have elaborate infilitrafion networks set up - tunnels, minisubs, etc. to smuggle men and contraband into South Korea. The Norks don't need missiles to hit Seoul - they just need a truck bomb. Can they pull it off? Maybe yes maybe no, but they're smart enough and advanced enough that that's a bet I wouldn't want to take.