One factor in favor of S. Korea and U.S.: North of Seoul used to be just another countryside. Not anymore. Heavy development projects made the region full of buildings and clogged roads. Such densely populated urban area, which sprung up in recent years, create a headache for advancing army. Buildings, especially apartment complex(many of them are made up of high-rise buildings) can be used as hand bunkers as defensive positions. Building can be demolished to create barriers.
Besides, there is far less flat area for providing armors and transportation vehicles to drive on. They will be trapped into a maze, taking fire from all direction.
Overdeveloped urban area can have some fringe benefit: to stop massed advance of enemy.:-)