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Photo: WatchWants at Korean Wikipedia, Public domain via Wikimedia Commons
The Great South Gate is an iconic symbol of Seoul and has been designated as National Treasure Number 1. Particularly beautiful when floodlit at night, and best combined with a visit to the adjacent Namdaemun Market. Much of the structure was destroyed by arson in 2008, although a faithful reconstruction was completed in 2013. Formally known as 'Sungnyemun' (숭례문, 崇禮門). The other city gates can be found in the neighboring Jongno district.
Namdaemun, a.k.a. the Sungnyemun, is the southern gate of The Eight Gates of Seoul making part of the Seoul City Wall marking the city's original boundary during the Joseon period, although the city has since significantly outgrown it after that. It is located in Jung District between Seoul Station and Seoul Plaza, with the historic 24-hour Namdaemun Market next to the gate.
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서울로 7017
A former motorway turned into pedestrian walkway. Opened in 2017, it offers a unique view above the cars and rails, with gardens and cafes along the path. Night view with car lights and neon signs is especially beautiful.
서울시립미술관
Renovated from an old court building, it mainly exhibits contemporary art works. You can see installments and video arts as well as paintings and sculptures. A cafe is with a souvenir shop on the first floor.
한국은행 화폐 박물관
You can find out about 20,000 rare domestic and foreign currencies held by the Bank of Korea and the money manufacturing process and the country's economic process.
The casino is only accessible to foreigners (it is illegal for Korean citizens to gamble), so remember to bring your passport. Various card tables and slots are available to play. There is a bar and cafe that serve food and drinks, although expensive.
문화역서울284
A culture center at the old Seoul Station. It was built in 1925 during the Japanese Colonial period, and the center of the Korean railway until 2004 when the current building for KTX opened. It reopened in 2011 as a culture center, and it hosts many exhibitions. The pre-modern architecture inspired by the Lucerene station (which has burnt down) offers unique sight among the glassy buildings of Seoul.
우표 박물관
It is a space where you can see the process of mail change and various stamps of the world.