Originally built as a secondary palace, Changdeokgung Palace is a nice place to check out, mainly for the secret garden. But that may depend on what time of year you visit, which can shape your opinion of this attraction.
If you have been to Gyeonbokgung, you may find them too similar and thus, this one is not as interesting. But it is on a smaller scale, since it was originally built as the secondary palace. It is still worth checking out, though how to do that, can be tricky. Here's why:
The Lonely Planet guidebook says that Thursdays are the only day in the week, where you can see the palace on your own, but I've heard a few people say that they did it on other days of the week. I don't know for sure, but if you have a choice, definitely see it on your own, as you get to see more at your own pace.
When entering the palace, definitely pick up a booklet at the entrance, which gives you a nice tour of the place and each area. Each area is named, and served a different purpose on the palace.
As for the secret garden, you can only be seen on a guided tour, with different times for different languages. What may sound like a tourist trap, the secret garden is surprisingly a nice place to check out, and the main reason to come to Changdeokgung. If possible, definitely visit in any season but winter, since the leaves on the trees, adds to the color of the secret garden, making the place look a lot nicer.
Get there early, if you don't want to wait for the secret garden tour after finishing seeing the palace, as it can be a long gap.
This review is the subjective opinion of a TravelPod member and not of TravelPod.com.