UNESCO World Heritage, one of the best fall foliage sightseeing spots in Seoul plus Moonlight Drawn by Clouds filming location- Changdeokgung Secret Garden
Changdeokgung Palace (especially its Secret Garden) is the most beautiful palace in my opinion, especially in autumn. I have been there 3 times. My friend who will be going to Seoul with me next month was not particularly eager to go until she realised it is the place Crown Prince Hyomyeong actually lived. Hahahaha.
Secret Garden requires reservation, and the system just changed in July, the reservation for tour starts 6 days before the day of visit. The quota is filled up quickly in autumn, please remember to reserve a place on the day it starts,
P.S. This is also the filming location for Ep 2 Moonlight Drawn by Clouds