There are tooooooo many craft beer places to list in Seoul now. The most famous ones are Craftworks and Magpie. Most of these craft beer bars are concentrated in Itaewon/Haebangchon/Gyeongridan.
A unique option would be Woori Super in Gyeongridan. This is technically an old convenience store that is converted into selling craft/imported beers only. Like too many to count and list and each time you browse you have to spend a ton of time. They have a very, very impressive selection. Outside of the store are tons of tables where you can sit and drink the night away and it's very lively. I definitely recommend it.
https://www.trazy.com/spot/3313/woor...estaurant-cafe