Few countries have their capital that close to a hostile border. Syria does. Look where Damascus is.
1) Artillery isn't actually that good at 'clobbering' a city. 2) North Korea doesn't have THAT much artillery, and even less of it is working, nor do they have the supply lines to keep them constantly resupplied 3) Seoul has quite a bit in the way of bomb shelters
Seoul has has 10 million people in it, and most experts think that at most 30k or so would die if NK was able to shell them.
That's certainly(!) nothing to scoff at, but it's not really being clobbered. The media likes to portray it as NK being able to completely level Seoul, which just isn't reality.
http://atlanticsentinel.com/2012/06/north-korea-cant-really-... is a pretty good article on this.
As with most forced urban planning efforts, the city is struggling to attract residents who aren't civil servants. The city is expensive to build, expensive to maintain, and due to budget cuts there is (still) no planned high-speed rail[2], although it seems they are at least connected to at least one existing "normal passenger rail" line that takes 90 minutes with trains leaving every 30mins[3].
[1] https://en.wikipedia.org/wiki/Sejong_City
[2] https://www.ft.com/content/ba9646f6-4dc6-11e3-8fa5-00144feab...
[3] https://en.wikipedia.org/wiki/Sejong_City#Transportation
The majority of NK's artillery is not long range and can't threaten the densest parts of Seoul; while those that are long range will be immediately targeted and destroyed once they begin firing. Then there are serious questions regarding the quality of NK's equipment and their troops, plus the fact that artillery on its own is not particularly lethal.
Finally, South Korean civilians in the part of Seoul most under threat are prepared for this attack and most every subway station can double as a bomb shelter.
Any NK artillery attack would definitely kill a substantial amount of civilians, but it wouldn't be 100,000s and it wouldn't destroy Seoul. The real wildcard is if NK uses chemical or nuclear weapons which changes everything.
There's a good summary of any potential NK attack on Seoul here: www.dailykos.com/story/2017/4/25/1656090/-North-Korean-artillery-and-the-concept-of-flattening-Seoul-a-breakdown
However it's a useful deflection from the fact that since the USA retired its enhanced-radiation weapons ( 'neutron bombs' ) in the 1990s, there is no prompt-strike way to negate several thousand hardened artillery emplacements linked by tunnels. Not without either hitting each individual emplacement or by using mass-effect weapons that in turn threaten Seoul.
Given that, I think it's likely that the USA-SK war-plan is to decapitate the North Korean command structure so quickly that the artillery doesn't even receive the order to fire.
Alternatively, that artillery gets clobbered in the first minutes.
TL;DR; DPRK can probably get off at least one good barrage, not enough to level Seoul but it would hurt.
N Korean army would be really idiotic to focus all their available artillery on Seoul and ignore all other military installations.
During the Battle of Britain, Luftwaffe nearly broke RAF when they concentrated on bombing the RAF bases. But when Nazi switched their focus to bombing London, RAF had a chance to recover and eventually beat back Luftwaffe.
And you can bet S Korean artillery units won't be sitting around when the balloon goes up.
The Kim family stepped in for Japanese Emperor as far as being divine father figure for the nation, at one level the 'world power' rationale is the same as what the UK had to play post WW2. Much like it has been in other parts of the world the enemy is the USA and it is quite clear that the goal of the North Korean programme is to be able to get some missile with some nuclear weapon at least as far as a big city in the continental USA.
I am no defender of the crazy kid, however, there is some serious stuff behind the attitude of North Korea. They do not want their culture to be Americanized, they do not want advertising and they do not want to be yet another consumer society. A whole smorgasbord of Western beliefs are just not wanted by them. There is also the matter of unification. Their idea is to have a federation, one country with two states, governed by their respective leaders and with the one seat for the federation at the UN. That is what they want and without any outside interference. So those nuclear weapons are pointing outside of the Korean peninsula and that would be the situation of a Korea united on the North's terms - 'without outside interference'. A unified Korea with weapons of the nuclear flavour would keep all the neighbours at bay on the MAD basis so a unified, federated Korea (which sounds reasonable to some in the South) is just not going to happen.
Seoul is not going to get annihilated by the North. Ignore the Kims for a moment and look at what the Koreans could achieve. A united Korea would be second on the stage after the USA in terms of GDP+population+trade+influence and if you add the nukes then they are totally deserving of taking the UK seat or anyone else's except the Americans in the security council. That might sound far fetched, but I believe that this is an obvious prize in Korea and destroying Seoul goes against that.