You may not have heard of many of these cities before, but these are modern suburban centers.
Everyone is living in high rise apartments
Excellent public transportation available - subway and buses
Deep, well-connected foreigner communities
Plenty of universities and colleges in every city, which brings young people, great nightlife and inexpensive dining options
You are close enough to Seoul to easily get there in less than hour, while also being able to escape the concrete for mountains and green space just as easily.