I’ve been in some of the underground shelters. Immense underground shopping malls during normal times.
NK is almost certainly going to use chem and bio weapons. Plus the burning buildings will cause significant problems.
The thing is, crack NK commandoes will lob Sarin and VX gas mercilessly into the Seoul subway stations, once they are full. They know every station, and have trained for it...I am certain they are already on SK soil, assleepers, as we speak.