This church is a famous historic site. It was a marker for where a number of Catholic martyrs lost their lives during the Joseon Dynasty. It was later moved to its current location so that it could be expanded.
The design of the church is a bit of a mash up of Byzantine and Romanesque. It is boasted to be one of the most beautiful structures in Korea because of its unique mixture of architecture. Personally, I think that statement is selling traditional Korean architecture a bit short.