Myeongdong Cathedral, Seoul

Also known as the Cathedral Church of Virgin Mary of the Immaculate Conception, the Myeongdong Cathedral clearly represents South Korea's Catholicism. Styled in Gothic Revival architecture, its red and gray brick facade is actually atypical of its kind but more like a fusion with Korean-style finish. The structure itself is quite outstanding (but not imposing), being built on a much higher ground than the rest of its adjoining structures. The grounds are well maintained and kept spotlessly "immaculate" despite the throngs of tourists that flock the area. The interior is equally magnificent with its high vaulted ceilings, pointed arches, and stained glass windows.

The street where Myeongdong is located comes alive especially during weekends so one can visit the shops and try the various street food that plies the alleyways.

Tip:
It might look like it's closed, but the doors either side of the main door are quite possibly open.
Read a bit of the history (free guides in many languages) as it makes it more relevant.