Alright, there were actually no scenes that was ‘really’ filmed inside (or even outside) the National Assembly, I believe. Just a POV of Jo Guk from his car and some studio-filmed scenes that was supposed to be the inside of the National Assembly (unless they really filmed it inside?).
Nonetheless, for Kim Sun Ah / City Hall fans (fondly called Cityhallers), the National Assembly = Congressman Jo Guk!
HOW TO GET THERE:
Take Line 9 and get off 국회의사당 station Exit 1.