We see this cathedral multiple of times in Black Knight: The Man Who Guards Me (흑기사) (KBS2, 2017-18) (In fact every time somebody goes to Sharon Tailor):
Episode 01, when Jung Hae-ra (Shin Se-kyung) was looking for an old tailor shop to get her dress in her ‘dream’.
Episode 02, when Hae-ra and her friend were looking for Sharon’s (Seo Ji-hye) Boutique.
Episode 03, when Hae-ra together with Moon Soo-ho (Kim Rae-won) are on their way to Sharon Tailor Shop.
It is again shown in episode 6.
Myeong-dong Cathedral [명동대성당] is located in Myeong-dong [명동], Seoul.
Address: 74 Myeongdong-gil, Jeodong 1(il)-ga, Jung-gu, Seoul, South Korea
서울 중구 명동길 74
Geolocation: 37.563240, 126.987217
To get here: Myeong-dong Station [명동역], Line 4, Exit 9 or 10 / Euljiro 3-ga [을지로3가역], Line 2 and 3, Exit 12
— submitted by ruffeecola
— Photo taken by © ruffeecola, [Nov 2016]. View more of her photos of the location here.