Supposedly more tubes (artillery & heavy mortar) that can deliver more explosives that were delivered by artillery ordnance then WWII. All sighted in (or nearly so!), all that can fire pretty quickly some say within 20 minutes.
I have seen these factoids in several places and I do find them a bit hard to believe. However be sure a huge targeting problem if one wishes to save Seoul & minimize casualties.
Even if it is only 5,000 tubes it is still a significant problem.
100k casualties in the first salvo if no warning in Seoul. 300-500k if no counter battery fire from South Korea. 1mm + if CBRNE employed. Gas is hell in built up urban areas, esp. if heavy and seeps down into bomb shelters.