We took the Seoul City Tour Bus, Circulation Course (free with your Seoul City Day Pass; W 12,000 adult/ W 8,000/child (2-story bus)) that you can hop on and off at 26 destination throughout the day and got off at N Seoul Tower, which was not on our itinerary but we got off on a whim. Supposedly, you can also get the N Seoul by cable car but we did not take that route. If you have more time, you can also take your time at Namsan Park which is also at N Seoul Tower (ticket price W 9,000 adult/W 5,000 child). Before the entrance, you'll see a banner saying that it's voted number destination by foreign travelers.
N Seoul Tower is not the tallest towers in the world by any means but it boasts a 360 degree view of downtown Seoul and Namsan Mountain, as well as a has a historical and cultural significant for the people of Seoul and so I think that is why it is the number one voted destination. Interestingly, I asked the elevator guy there and my Korean friend as to the meaning of "N", and no one seems to know. If you're going to the top, make sure to stop by the "toilet" as it will surprise you with a complete open view of the outside. Bring padlocks if you're lovers who want to be tied together at the love pad lock trees.