Last fall I took my mom to visit Changdeokgung Palace when she came to visit Korea. I had visited the palace on a previous tour, but not during fall. At the time, the fall leaves were just a bit past peak color, but they were still vibrant enough to make you stop and stare.
We got to the palace relatively early, because we had a full day planned of palace visiting and walking around Seoul. We first did the Secret Garden tour, which is to the rear of Changdeokgung Palace, and actually makes up the bulk of the complex.
You can only access the garden via a guided tour, of which there are multiple ones throughout the day, in both Korean and English. However there are only two tours in English, at 1130 and 1430.
The garden was designed for the private pleasure of the king and his family. It has a wide variety of buildings, gardens, ponds, flowers, and trees, some of which are hundreds of years old.
The highlight of the Secret Garden tour is the Buyongjeong Pavilion on the Buyongjeong Pond, which was restored a couple of years ago.
The tour of the Secret Garden lasts about two hours, and it moves at a leisurely pace. You definitely have plenty of time to enjoy the scenery, listen to the tour guide and take pictures.
This was definitely one of the most beautiful places to see in Seoul in the fall. There are many places that have colorful leaves, but the Secret Garden has some of the greatest concentration of fall beauty.
Changdeokgung Palace is easy to get to, since it is centrally located in Seoul. You can reach it via metro line 3 (Anguk stop, exit 3), and it is a few hundred meters up the road, or you can walk to the palace from metro stop Jongno-3(sam)-ga (Lines 1,3,5, exit 7). There are separate English language tours for the palace (though you can walk through most of the palace grounds on your own) and the garden (guided tour required). It is open from 0900-1700 (winter) or 1830 (summer).
If you like palaces, and especially if you love beautiful gardens, this is definitely a palace to visit. And if you have the opportunity to visit it in the fall, that is even better.