It is as simple as looking at a map.
If you go to the website linked below you can get a zoomable version.
http://images.google.com/imgres?img...q=Souoth+Korea+map+population&hl=en&sa=G&um=1
The most advanced artillery systems of the world (which NK doesn't have but we will give them the benefit of the doubt) have on the order of 30-35km max range using rocket assisted ammunition. The center of Seoul is at right about 40km from the closest point in NK, with the majority of the city South of that point and the Han river.
The above means that even with world beating artillery the vast majority of the city is not in range of NK artillery at all. But more than that given how antiquated the NK military is and the fact that only the heaviest/most advanced tubes would be at the 30-35km range anyway, only the farthest Northern reaches of the city are vulnerable to concentrated fire. On top of that, the NK artillery is not all concentrated at a simple point on the NK border but deployed several miles deep into their own territory where there are suitable defendable locations.
In other words, the idea that Seoul would be "leveled" is ridiculous.