Thorgalaeg
Deity
North Korea knows it cant win such a war and all his strategy is based on taking Seoul as hostage. It will probably put all his weight in taking the city in the first hours not matter the price. And last time i checked NK surpassed SK in infantry and number of tanks amply.I also don't see North Korea achieving a breakthrough of South Korea's defenses in a matter of hours, nor do I see what you are basing such an assumption on. I think you are putting too much stock in North Korea's numbers (which really aren't that much larger than South Korea's by the way), and not factoring in all the force multipliers that South Korea and the US have at their disposal. With the US launching constant disruption operations behind the front lines, North Korea wouldn't be able to focus their air defense at the front, so while South Korean troops hold them back from carefully prepared fortified positions, North Korean ground troops would be getting constantly harassed by US, South Korean, and possibly Japanese aircraft. That would make it very difficult for North Korea to break through quickly and the fighting would almost certainly devolve into a WWI-style war of attrition as North Korean forces would be forced to dig in to protect themselves from the constant airstrikes.
It would also be very stupid for North Korea to focus their artillery strikes on Seoul, as those guns are certainly going to be needed to help break South Korean fortifications at the front. Firing at Seoul instead of fortifications would leave those tank divisions of theirs totally exposed and without fire support, which would be a death sentence for them. Plus, we have these really cool counter-battery systems installed on our artillery now, so every time one of their guns fires, one of ours will fire right back at it and destroy it.
Dont say it will break through ROK defenses but there is many chances it will do, or at least will try by all means, and in such case nukes would be the only way of saving Seoul.