The Lotte Tower Seoul Sky observatory should certainly be listed as one of the highlights of a visit to Seoul. The views, especially of the Han River, at sunset, and just after sunset are breathtaking. You can also see the Olympic Park very well. Admission at 27,000 won is reasonable compared to other World Class towers. There was no wait when we visited on at 6 pm Monday evening. The surrounding area is also of interest, for example Lotte World. There was a haze but it wasn't too bad.
Seoul is a fascinating mix of its ancient civilization, the devastation and its unflinching remembrance of its bitter first half of the 20th Century, and the current relentless vitality, affluence and modernity of today. Along with the frenetic shopping districts, the Lotte Tower is an iconic symbol of the latter. Don't miss it !