It's hard to describe Seun Sangga in a few sentences. If you're interested in 'the past and the present' of Korean architecture, please just go to Seun, not DDP. It's a former electronic shopping mall/apartment from the 60s, recently ('17) redeveloped by Seoul City Government. But since it's a large complex consisting of 7 buildings overarching Jongro-Euljiro area, a bit of research on Google is required. I'd say the best way to explore it would be visiting an art space 'Open Circuit' and start from there (It has a FB page). Now it has an above-ground pedestrian bridge and a rooftop (9th floor) where you can get a nice view of the city.
175-4 Jongno 1(il).2(i).3(sam).4(sa), Jongno-gu, Seoul, South Korea