From a variety of sources we have some interesting images, North Korean Peoples Army [NKPA] artillery as positioned along the De-Militarized Zone [DMZ] separating North [DPRK] and South Korea [ROK], the preponderance of that NKPA tube artillery NOT able to range and fire on the very downtown of Seoul.
That "Sea of Fire" as threatened by NKPA military planners including of course Seoul AND environs.
ENVIRONS extending from what distance outward from the city [Seoul] outward I cannot say.
NOTE these images not including NKPA rocket artillery of the 240 mm caliber. The NKPA also possessing an antiquated [but still deadly] mobile multiple rocket launcher [MRL] of 200 mm caliber the range of which is significantly [?] less than the 240 mm MRL version
Of all that NKPA tube artillery arrayed on the DMZ ONLY the Koksan 170 mm gun caliber able of ranging as far as Seoul and not even that to the downtown section of the city. NO rocket artillery of the 240 mm caliber included in this image.
Seoul in the lower right hand of this image the Koksan gun able to range to the very outskirts of the city but not beyond. Again this not including rocket artillery fire of the 240 mm caliber. The metropolitan area of Seoul very large and very densely populated, Seoul and environs the target of NKPA weapons fire could cause a mass panic?
NONE of these images taking into account the ranges and numbers of NKPA heavy mortars [caliber of greater than 122 mm]. In the Soviet scheme of things [and as followed by the NKPA but not slavishly so] mortars of large caliber classified as "artillery".
That NKPA tube artillery during the initial stages of a conflict firing from protected positions established long time ago, the ranges to potential targets determined also well in advance. NKPA artillery and MRL mobile displacing further south of course WELL ABLE en masse to range Seoul with ease, that "sea of fire" not only a possible then but a given.
And if and when chemical and perhaps biological rounds used, all bets off!!